Major League Baseball is testing a check swing challenge system in the Arizona Fall League:

Teams have two check-swing challenges in the game. Per the new rules, only batters, pitchers and catchers can invoke an official review.

However, managers and catchers can still appeal a potential check swing to first or third base umpires as normal.

Should a team utilize both challenges before the ninth inning, the club will receive an additional challenge to use for the last frame. The official review will determine if the hitter’s bat crossed a 45-degree threshold from home plate.

KansasCity.com

I think it’s great, the players seem to like it, and it finally gives us a clear definition of what is and isn’t a check swing.