MLB debuts automated check-swing challenges: Will it reach the big leagues?

The AFL is no stranger to being Major League Baseball's testing ground. In 2019, MLB began testing the automated ball-strike system in AFL games. While the "robot umpire" has yet to make its debut in the big leagues, other initiatives began their life in the AFL and were ultimately implemented — the most recognizable of these being the pitch clock. 

The use of motion tracking technology to review a check swing could also mark a major change for baseball as there is nothing in the official rulebook defining what a check swing is. Only umpires are allowed to determine what is or isn't a swing altogether. 

There are general signs that a player swung, such as their wrists rolling over or the bat crossing the front plane of the plate, but an official ruling on how far the bat must go to be considered a swing has yet to be written. The use of this technology would have to create the definition of a swing for a challenge to determine if the checked-swing strike call is correct. 

The technology got another chance to be used later in the game when New York Yankees prospect Garrett Martin also successfully challenged a checked-swing strike call. Martin's swing was nearly parallel to the 45-degree line, but did not cross it.

"They show a dotted line on the screen, and if the bat touches that dotted line, then it's a swing," Scottsdale manager Dennis Pelfrey explained to Baseball America senior editor Josh Norris. "And that's a 45-degree angle from the knob of the bat through the barrel, with home plate being the zero line." 

As explained by Norris, the animation on the video board shows a side view of the swing path then an overhead view of where the bat stopped in relation to a 45-degree angle. If the bat's barrel crosses the 45-degree line without hitting the ball, it's a regular swing and a miss. If not, the player successfully checked their swing, and the strike call depends on where the ball landed. 

Considering the angle, that's a generous amount of room for a hitter to halt their swing. 

As of this writing, MLB hasn't provided a reason for using a 45-degree angle to determine a swing, but it could change as the review system is tested further. This change likely aligns with recent efforts to give hitters an edge over historically tough pitching.

When asked about the advantage o the 45-degree angle, Arizona Diamondbacks prospect Tommy Troy told PHNX Sports Diamondbacks beat reporter Jesse Friedman that it's a pretty generous set up for hitters, but he wasn't upset by it. 

"It's hard enough to hit," he said with a laugh. "Why not?"