Ending to Brewers' Game 2 World Series win shines spotlight on pitfalls of new check-swing system

Game 2 of the World Series between the Milwaukee Brewers and Tampa Bay Rays delivered no shortage of drama and fireworks. Milwaukee erased another 4-0 deficit to secure a 5-4 victory, Sal Frelick launched a game-tying home run off a left-handed pitcher, and rookie Cooper Pratt delivered a go-ahead RBI single after committing a costly error to open the game. Manager Pat Murphy also made a notable bullpen decision, sticking with Aaron Ashby for a multi-inning save rather than turning to Trevor Megill to close things out.

On Tampa Bay’s side, several costly moments contributed to the loss. Third-base coach Brady Williams sent catcher Liam Hicks home on a play that resulted in an easy out at the plate, first baseman Jonathan Aranda committed two defensive miscues, and Gold Glove finalist Chandler Simpson saw the ball pop out of his glove when trying to complete a diving catch.

Additionally, the Rays’ final chance to tie the game in a dramatic ninth inning came up just short when DH Yandy Díaz struck out on a check swing, leaving the tying run stranded at third base. Whether Díaz actually went around remains a point of debate among fans, but first-base umpire James Hoye ruled that he swung, and the Brewers won the game.

Nevertheless, the controversial ending raises a question about how a check swing is defined and whether it could eventually be subject to a review system, much like balls and strikes are now. The technology exists for an "Automated Check-Swing" challenge system (ACS), and it's currently being tested out in the minor leagues. ACS was first implemented in the Arizona Fall League in 2024 and has since been used in the Florida State League (Low-A), and the Pacific Coast League (Triple-A).

While ACS is not yet used in MLB, one social media account applied the technology to Díaz's check swing in Game 2 of the World Series, and inadvertently pointed out a flaw with the check-swing challenge system.

MLB may need to rethink the guidelines of the Automated Check-Swing challenge system (ACS) before implementing it

The popular X account "Tangotiger" shared a clip of Diaz’s final swing from Game 2 of the World Series being run through the current ACS system. The result, according to ACS, was that Diaz did not swing. However, the parameters used within the system to make that determination are important, as ACS uses a 45-degree mark as the cutoff for whether or not a swing occurred.

Defining a check swing using a 45-degree angle is an interesting approach, particularly because that mark seems like much more of a swing than many would traditionally consider a swing. Additionally, attempting to measure something that MLB’s official glossary currently offers no formal definition of is challenging. Currently, a check swing decision is up to umpire's discretion, with them relying largely on unwritten guidelines such as whether the bat crosses the plate or the batter’s wrists roll over, to determine whether or not a swing has occurred.

In Diaz's case, the bat very clearly crossed home plate, but didn't come anywhere near the 45-degree angle used in the current ACS system. Going forward, Diaz's strikeout should be used to help establish a middle ground in what is defined as a swing on replay, especially if MLB wants to incorporate this technology and player challenging abilities in the future. As the ACS system currently stands, it's implementation would completely change what is currently considered "not a swing," according to the unwritten rules. Is that a step MLB really wants to take or should the ACS guidelines simply be changed?

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