A batter tapping their helmet was the signal for challenging a call when the Automated Ball-Strike System was used during spring training in Major League Baseball. The system is still in use in minor league games, but is not currently being used in regular season games in MLB. used.

Taylor Walls was ejected in the ninth inning of the game after tapping his helmet. Here's why the umpire saw it as a violation.

During spring training, MLB made the decision to test an automated strike zone (ABS) with a ball-strike challenge system for a majority of Grapefruit and Cactus League games. And to signal a challenge, a batter would tap on his helmet after a pitch. Simple enough.