immediate signal by base umpire

RulesMLB Umpire Manual

“VOLUNTARY STRIKE”

Rule 8.02(c) Comment: In the situation where the third strike eludes the catcher on a half-swing and the batter-runner is entitled to run to first base, the appeal should be made to the base umpire instantly (without waiting for a request from the defense); but even if the appeal is not instant, the appropriate base…

Don't strike out!

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