One thing that’s been endlessly debated among baseball people: Exactly what is a “check swing”?
In some games in the Arizona Fall League, MLB is now testing a challenge system that aims to specifically define that.
This article by Sam Dykstra at MLB.com explains how it works:
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.