It’s one of the great sports debates – was that a swing?

Every baseball fan has participated in the discourse at one point or another. You’re watching a game when a pitcher throws, say, a nasty changeup that dives below the zone. Fooled only briefly, the hitter begins to swing but checks himself just in time. Or did he? The home-plate umpire can’t tell, calls a ball and the catcher points to the first-base or third-base umpire for appeal. Then fans see multiple camera angles and slow-motion video that the umpire doesn’t, and the debate rages.

Let’s face it, the check-swing call is one of the most subjective in baseball. Major League Baseball is now beginning preliminary testing of technology to gauge whether there is a way to create more clarity on check swings.

Using the same Hawk-Eye technology as the Automated Ball-Strike Challenge system already in place in the Arizona Fall League, the Check Swing Challenge is being used on a limited basis in a proof-of-concept stage at Salt River Field.

Teams will be able to challenge an on-field check-swing call to see if the swing crossed a 45-degree threshold from home plate.

That’s the elevator pitch. Here are more details on the latest review system coming to games at Salt River and why it’s being tested in the weeks ahead.

What is a check swing?
There is no definition for “check swing” in the Official Baseball Rules. The closest thing comes in the Definition of Terms where the first example of a strike is a pitch that “is struck at by the batter.” It’s up to the umpire to decide what “struck at” means. Rule 8.02(c) gets into how managers and catchers can ask for umpire help on “half swing” calls but does not expand from there.

Without official definitions, we’re left in a gray area. Common understanding is that the batter has to “cross the plane” or “break his wrists” in a check swing for it to be a strike, but that’s still at the umpire’s discretion without an official rule to fall back on.

So how would this change?
Under the Check Swing Challenge system, managers and catchers still ask for appeals, and umpires still make an initial call on the field. If players disagree, they can challenge the check-swing call to Hawk-Eye, bringing a replay to the in-stadium video board.

The check swing is called an official swing (and therefore a strike) if the review reveals that the bat exceeds a 45-degree threshold from home plate. Want an easier way to visualize that? The foul line is a 45-degree angle from home. If a right-handed batter swings beyond the first-base line, it’s a strike. If a left-handed batter swings beyond the third-base line, it’s a strike.

How many challenges per team per game?
Each team gets to challenge two check-swing calls per game. If a team uses up those two challenges by the ninth inning, it is granted one additional challenge for the final frame.

Who can challenge?
This is the same as with an ABS Challenge. Only pitchers, hitters and catchers can ask for an official review.

Where will this be first tested?
The Check Swing Challenge will only be used in games played at Salt River Fields at Talking Stick in this year’s Arizona Fall League for now. Salt River serves as the Spring Training home of the D-backs and Rockies. This fall's Salt River Rafters roster is made up of players from those two systems as well as from the Nationals, Twins and Yankees.

Does the Arizona Fall League have a history of testing out potential new rules?
Yes! ABS, pitch clocks, bigger bases, mound visit limits, shift bans – many potential rule changes (both big and small) have been tested in the AFL before they’ve made (or, in some cases, not made) the Majors.