Run through first base. It's one of the most fundamental principles of baserunning.

Run through first base, because that's how you get there the fastest, and you're protected under MLB's Official Rule 5.09(b)(4): "A batter-runner cannot be tagged out after overrunning or oversliding first base if he returns immediately to the base." Slide into the other bases, where no such protections are afforded. Baseball 101.

But what happens when baserunners start running through second?

Players around the Major Leagues -- including on many of the teams contending for the 2023 postseason -- are employing that exact baserunning strategy: They are running through second base (or even, rarely, third base), instead of sliding.

This is a play that, if a team can pull it off, might swing the balance of a playoff game. Here's how it works.

• The most common version happens with runners on first and third, or the bases loaded, and two outs. The batter hits a ground ball, usually to one of the middle infielders, who throws to second to try to get a forceout to end the inning.
• If the play is going to be close, the runner from first will run through the second-base bag to get there a split-second faster than if he slides, to try to beat the force play. By overrunning second base, he'll likely get stuck in a rundown and eventually be tagged out. But: A run will score. The instant that runner touches second base, the force is removed, which means the run coming home from third will count, so long as that runner crosses the plate before his teammate who ran through second is tagged out.

The third out is probably going to happen either way. But if that out comes by a tag play instead of a force play, you can push across an extra run. And one more run can mean winning the game. So more and more, you will see big league teams try running through second base in this situation.

"You're really just trying to steal a run there," said the Blue Jays' Cavan Biggio, who's among the players to have attempted the play this season. "If you run through the bag as if you're running through first base, it's a little bit quicker. If you can beat it and get the runner to score and then they tag you out, that's better than just sliding into a forceout."

"Everybody knows the general concept of beating the ball to the spot, right?" said Cubs rookie Pete Crow-Armstrong, who tried the play on Sept. 16 in Arizona, his fourth big league game. "How I see it is: You could create contact, somehow force a fumble. And you put the pressure on the defense, seeing you go full-head-of-steam into them. I like the thought of that little added pressure. What I learned is, you're not automatically out if you go out of the baseline after that play happens, right? You're not out until they call you out. So it's such a smart play if you can beat the ball there."

How did this become a growing trend in the Major Leagues?

The Cardinals are in large part responsible. On June 14, 2022, Nolan Gorman threw a game into chaos when, with two outs and the bases loaded, he sprinted through second base on Paul Goldschmidt's chopper up the middle. Gorman beat Pirates shortstop Diego Castillo's throw to second, turned the corner and kept on going.

What ensued was one of the wackiest baserunning sequences of the year, a series of rundowns that ended up with Edmundo Sosa tagged out at the plate to end the inning. But Yadier Molina had already come home to score. The impact was made the second Gorman's foot hit the second-base bag ahead of the throw.

"The front side of that play is probably one of the most heads-up baseball plays you’ll ever see," Pirates manager Derek Shelton said after that game. "It was Gorman, right? Ran straight through the bag."

Around the league, other teams took notice of Gorman's heads-up decision. Now they're following suit. The Dodgers, Rays, Brewers, Blue Jays, Cubs and Marlins are all among the teams in the 2023 postseason race who have used the run-through-second strategy this season.

"It's a little strange, but I think with the analytics of baseball now, they're proving that it's going to help possibly win the game," Rays infielder Isaac Paredes said, via team interpreter Manny Navarro. Paredes has attempted the play as a runner, and also had it tried against him playing second base.

"I'm sure that it's probably leaguewide and sport-wide that people are starting to take notice," Crow-Armstrong said. "We'll probably start seeing it practiced a little bit more in Spring Training and whatnot -- but that's just speculation."

Gorman isn't the first baserunner to run through second base to try to beat the force -- several veterans who have done it, like the Dodgers' Jason Heyward and the Marlins' Jon Berti, said they've been aware of the play for several years. But the high-profile success of Gorman's play has had a clear influence on Major League baserunning technique.

Multiple players from other clubs that now employ the same strategy cited Gorman and the Cardinals as the starting point for their own team's adoption of it.

"St. Louis did it last year and was able to score, and ever since then, that’s the play," Brewers first-base coach Quintin Berry said.

"That’s why it’s hard to have secrets -- because it gets picked up," Milwaukee manager Craig Counsell said. "Especially in the video era, anything new is going to be on video the next day."

That's how Crow-Armstrong learned. He watched Gorman's play, and then suddenly, the Cubs organization was laying out the strategy, from his bench coach in the Minors with the Tennessee Smokies, Marco Romero, to his current first-base coach with the Cubs, MLB veteran Mike Napoli.

"I was still in the Minor Leagues when I saw it for the first time, but it was the Cardinals that I saw do it," Crow-Armstrong said. "Even back in Double-A, when I saw that, we talked about trying to make stuff like that happen."

When the 21-year-old Crow-Armstrong got called up to the big leagues, Napoli reiterated the play to him -- immediately before the rookie did it.

"It's funny, man, Nap -- I've learned a ton from him," Crow-Armstrong said. "He goes, 'Hey, we haven't really talked about this yet, but if there's a ground ball hit up the middle, you're going through the bag running.' I'm like, 'Yeah, I know what we're talking about here.' … And it literally happened the next pitch. It's little moments like that that help me gain info and experience on future baserunning plays I'm gonna be making."

Even Berti, who said he's tried the move several times throughout his career, is on a club influenced by the Cardinals. Miami was teaching the strategy in Berti's first season there in 2019, but it's now being reinforced by first-year manager Skip Schumaker -- who was the Cardinals' bench coach when Gorman's play happened.

"So, I was taught this before," Berti said. "I think [then-Marlins first-base coach] Trey Hillman was the first one, my rookie year, to mention it to me. [But] then also in Spring Training, Skip went over it, too, that they had done it before in St. Louis."

The running-through-second trend is not always easy to spy in action. The opportunity to try the play arises infrequently, and you have to be paying close attention to notice it, because the end result is often a routine, inning-ending forceout. To succeed -- to bring that run home -- is even rarer.

But that one time it does work, it can alter the course of a game.

In back-to-back games this month, the Yankees successfully used the play to score key runs. On Sept. 14 against the Red Sox, Oswaldo Cabrera raced straight through second on DJ LeMahieu's two-out, bases-loaded bouncer up the middle, a play similar to Gorman's. Boston second baseman Pablo Reyes had to go to first instead of second, LeMahieu beat the throw, and a run scored.

The very next at-bat, Aaron Judge crushed a grand slam.

"If we see an opportunity to be safe at second, we try to go straight," Gleyber Torres said. "When you slide, you've got more chance to be out. If you go straight, you've got a little more chance to be safe."

A day later, Estevan Florial was the Yankees runner who made something happen in New York's ninth-inning rally against the Pirates. With the bases loaded and one out, Anthony Volpe hit a potential game-ending double play ball to short. But Florial, bearing down on second baseman Ji Hwan Bae, forced Bae into a throwing error. The go-ahead runs scored and the Yankees ultimately won in extra innings.

"I think guys coming up now didn't play with that [pressure on the fielder at the bag]," Yankees manager Aaron Boone said. "When I played, you were doing all you could to disrupt that and get after that second baseman. For the second baseman, one of the job requirements was: Can you protect yourself at second base? In Cincinnati, I moved from third base to second base one year for an up-and-coming player. One of the things I had to spend the whole winter on was making sure I could protect myself at second on that pivot."

That echoes something Biggio remarked about what his dad would think about running through second -- that's Hall of Famer Craig Biggio, who played in Boone's era and was both a savvy baserunner and a second baseman who had to deal with runners coming hard at him.

"Knowing him, he'd probably appreciate it," Cavan said.

Paredes, having been on both sides of the play, also provided the fielder's perspective, saying: "You just feel close -- very close. Kind of like you've got to rush or get out of the way."

The Yankees' frequent recent use of the baserunning play has drawn attention. After Torres went through second standing up on Sept. 19, former MLB slugger Vernon Wells posted about the play … somewhat incredulously.

But the Yankees are only one of many teams employing the strategy.

The NL West champion Dodgers, who are approaching 100 wins and will enter the playoffs as the No. 2 seed in the National League, use the technique consistently -- players like Heyward and Max Muncy have attempted it several times.

"It's about playing the game all the way through and paying attention to detail," Heyward said. "[You do it] every opportunity you get. Every chance you get."

This is a play that teams will put on, not just a choice by individual baserunners -- especially when that run at third base is extra important, like late in a close game. The first-base coach is often responsible for alerting the runner that the opportunity to run through second is there.

"You've got to put it in your head before the play happens," Biggio said. "Our first-base coach, [Mark Budzinski], does a good job of that."

"At the beginning of the year, [Rays first-base coach Chris] Prieto talked to all of us about the possibility of that play, and the importance of running it," Paredes said. "Anything can happen. It rushes the infielders. It can help score a run. It can put a little bit of pressure on them. Just like when you're getting in a rundown, it's just something that can change the game."

Running through second base successfully, however unlikely, has a high upside. But it's not completely risk-free. The runner must avoid a collision with the fielder as he charges through the base.

Biggio expressed a worry about trying the play with one out, for example, because you could be called for interference, resulting in an automatic double play. And as Boone put it, you have to "measure the danger of the play" and factor it into your baserunning decision.

"It’s hard to do," Counsell said. "It's hard to get a piece of the base where you can feel comfortable running full-speed with that guy standing there and not run into him. Because you can’t run into him."

Once the baserunner has decided to run through the base, the biggest thing he has to do is pick a lane. Depending on where the fielder is positioned on the bag, the runner will either have to take a tight turn around second base and go toward third like he would advancing on a base hit, or go straight through the base like he's running through first and continue toward the outfield grass.

Teams like the Brewers, Rays and Cubs all practice that.

"Nap actually mentioned that to me," Crow-Armstrong said. "That was the one thing I could've done a bit better. If the second baseman's coming across the bag, you want to pick the outside lane, go to the outfield side of second base. And vice versa. If the shortstop's coming across the bag, you probably want to hit the inside, infield side of second base."

There can be close calls. Crow-Armstrong sideswiped D-backs second baseman Ketel Marte on his way through the base. And when rookie Milwaukee speedster Brice Turang attempted the play (he was initially called safe, but the ruling was overturned through a challenge), he barely avoided Pirates second baseman Vinny Capra, having to take a last-second turn toward third instead of running straight through.

"If I would have really run through it, I would have run him over," Turang said. "That’s where it’s kind of an ordeal. He was on top of the bag, on the back side of the bag, so I had to cut inside of it. But if they’re playing it like a first baseman, that’s perfect."

On the other hand, the close interaction of the runner and fielder also adds to the difficulty of the fielder making the play. Beating the throw clean is the most straightforward way of making the run-through-second strategy work, but forcing an error is just as good.

Even teams you haven't seen it from yet have it in their playbook.

"We had that play lined up [last week] with the bases loaded," Phillies first-base coach Paco Figueroa said. "It was [Brandon] Marsh and [Bryson] Stott ... They were both going to do it."

Figueroa was alerted to the play, and that a runner can legally do it, relatively recently. But he has the vision for how to execute it.

"Keep running to left field," Figueroa explained. "Now, if you’re going to for sure make it [safely anyway], don’t just give up an out for no reason. But it’s a great play. It’s actually genius to try to steal one run."

The consensus is: If you can do it, it's worth it. And it would be worth even more in a postseason game. So be on the lookout.

"If it's super, super close, and you beat that throw by sprinting through it, and then you're out: So what?" the Blue Jays' Danny Jansen said. "You give yourself up to get the run in. Any way. Winning just one run is huge."

"Just seeing how big of a possibility that play is, you've always got to be ready for it now," Crow-Armstrong said.

"I'm busting my [butt] to try to beat it out," Turang said. "I just want him to score. We know someone's going to get out unless there’s a bad throw or something. But I'm just trying to get that run to score. One run matters so much. One run can change a game."

MLB.com reporters Jordan Bastian, Adam Berry, Christina De Nicola, Bryan Hoch, Adam McCalvy, Sarah Wexler and Todd Zolecki contributed reporting to this story.