A pitcher must visually indicate with which hand he will use to pitch prior to the start of a plate appearance. This can be accomplished simply by wearing his glove on his non-throwing hand and placing his foot on the pitching rubber. Barring injury, he is not permitted to pitch with the other hand until the batter is retired, becomes a baserunner or is removed for a pinch-hitter.
The pitcher is allowed to pitch with the opposite hand mid-plate appearance should he incur an injury with the other, but he is then barred from pitching with the injured hand for the rest of the game.
Watch: Ambidextrous pitcher Pat Venditte declares the hand he will use to pitch with before facing switch-hitter Blake Swihart.