is that it's essentially an option, or a token, that can be played once at the beginning of the game if the manager so chooses. That token enables him to replace the pitcher's AB's with a 'designated hitter' but there's nothing compelling the manager to do so. If you want to pinch hit a pitcher for the designated hitter, or if you want to move the designated hitter off to a fielding position, the DH token is lost for that game and the pitcher now has to take his own AB's. So if the pitcher pinch hits for the DH, the DH slot transforms irreversibly into the pitcher slot. But you could have a pitcher come out of the bullpen, or off the bench, so long as it's his first action in the game and pinch hit for the DH (swapping him in for the current DH) and, so long as that 'pitcher' doesn't actually pitch, he's just a plain old DH.