Asking BU on a Check Swing

[Dave Reed]

Since there seems to some variance in opinion, here is what the MLBUM says:
8.7 "VOLUNTARY STRIKE"
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 umpire should immediately and voluntarily make a call of strike IF the base umpire is going to reverse the plate umpire's call. This will give the batter the immediate opportunity to run.

[Bolding, caps, and quotation marks copied from MLBUM.]