How does the Automated Ball-Strike System work? Stadiums are outfitted with cameras that track each pitch and judge whether it crossed home plate within the strike zone. While the strike zone actually called by big league umpires tends to be oval in shape, the ABS strike zone is a rectangle, as in the rule book. MLB has changed the shape of the ABS strike zone several times. Big league umpires call roughly 94% of pitches correctly, according to UmpScorecards.