Well, that didn't take long. See my earlier post for some context.

the Professional Baseball Umpire Corporation (PBUC) released its official rules for dealing with ambidextrous pitchers on Wednesday....

At the heart of the new guidelines is the following provision:
The pitcher must visually indicate to the umpire, batter and runner(s) which way he will begin pitching to the batter. Engaging the rubber with the glove on a particular hand is considered a definitive commitment to which arm he will throw with. The batter will then choose which side of the plate he will bat from.