The batter swings and pops the pitch up high into the air, over the infield. Multiple fielders gather, but there’s a miscommunication over who is going to catch the ball. Instead, the ball simply drops to the field.

A lucky break for the batter, right? What should have been an easy out now seems to be a hit.

Except, in this case, the batter is still ruled out. The reason? An obscure but necessary baseball rule that occasionally finds itself in the spotlight. It’s called the infield fly rule.

How it works

Some very specific conditions are required for the infield fly rule to go into effect.

1. There must be runners on first and second base, or the bases must be loaded (runners on first, second, and third).

2. There must be no outs or one out in the inning.

3. The batter must hit a fly ball or popup (not a line drive), over fair territory, and in the vicinity of the infield. It is not a requirement that the ball not reach the outfield grass. Rather, the umpire must use his judgment as to whether an infielder -- or the pitcher or catcher -- could make the catch using “ordinary effort.”

If these conditions are met, the umpire verbally declares the infield fly rule to be in effect and points to the air, while the ball is still airborne.

When this occurs, the batter is automatically out, regardless of anything else that happens on the play. The fielders can catch the ball, try to catch it and fail, or intentionally let it drop. None of that changes the result for the batter.

As for the baserunners, they can advance -- at their own risk. If the ball is caught, a runner who strays off his or her base risks being the victim of a double play, should the fielder throw back to that base before the runner returns. If the ball drops, a runner has the opportunity to make it to the next base. In this case, the defense must tag the runner out, because there is no force in effect.

In most cases, the runners will simply hold, and the play will end with the batter out, and the runners at their same bases.

Why it exists

You might be wondering why this highly convoluted rule is necessary. Well, imagine this scenario, in which the infield fly rule does not exist:

The bases are loaded with one out -- a great scenario for the batting team, which has a chance for a huge inning. But the next hitter gets under a pitch and lifts a sky-high popup toward third base.

All three runners know this ball is very likely to be caught, and that, if it is, they will be in grave danger themselves if they do not stay put at their respective bases. Therefore, nobody takes more than a few steps while the ball is in the air.

This gives the opportunistic third baseman an opening. He camps under the ball, but at the last second, lets it drop. He then picks it up, steps on third base to force out the runner on second, then fires to second to force the runner on first. What should have been one out becomes a double play that ends the inning and the scoring threat.

This is the reason behind the rule -- to prevent the defense from deliberately missing a catch in order to take advantage of runners who are trapped.

This situation does not apply if there are no runners on base, or the runners on base cannot be forced. It also doesn’t apply if there is only a runner on first, because if he or she is forced, the batter still has time to get to first, leaving the batting team in the same situation.

Gray area

The most well known, significant and controversial implementation of the infield fly rule took place in the first National League Wild Card Game, in 2012. And it illustrates how the rule, as necessary as it may be, isn’t always black and white. It relies on human judgment.

In this case, the Braves and Cardinals were playing in Atlanta, in a win-or-go-home game to determine who advanced to the NL Division Series. St. Louis led, 6-3, but Atlanta had runners on first and second with one out in the bottom of the eighth inning, looking to rally. On a 3-2 pitch from Mitchell Boggs, the Braves’ Andrelton Simmons lifted a pop fly to shallow left field.

Cardinals shortstop Pete Kozma drifted back, well on to the outfield grass, while left fielder Matt Holliday moved toward him. Each thought the other had called it, and neither made the catch. With the ball dropping, it appeared the Braves would have the bases loaded with one out.

The only problem was that the infield fly rule had been called. Simmons was out, and Atlanta had runners back at first and second with two outs. The Cardinals then brought in pitcher Jason Motte, who issued a walk to load the bases before getting out of the inning. The Cardinals won, 6-3.

Some thought the rule was applied correctly. Others argued it should not have been put into action on a ball hit that far. The only certainty is that there can’t be a definitive answer on something so subjective.