Not sure how long it's been in place - first year calling in FL (was in NC and NH before that). Essentially as it's been explained once removed from the mound the player marked as P/DH cannot enter defensively. Thankfully I haven't seen it in use yet. The whole removal thing is really odd, but essentially you end up inserting the defensive player for whom the (now) former P will be the DH (whether that player pitches or plays some other position). The "spot" in the lineup is now known as the DH spot. I cannot wait to see the coach that removes the P/DH and then reinserts him. My lineup card will become one big mess. I have to wonder though by reinserting him that means you have to burn another substitute for whom he returns as the DH for - since you're reinserting the sub in the lineup spot of the P/DH would now have to come out. If you don't have such player, then you have to play w/ 8 defensively since the P/DH cannot enter the field defensively after being removed. Probably only an issue in some extra inning game where one team has come back from a large deficit and the P/DH coach is now scrambling to get his better players back in the game.
This is how it's written:
• 3-1-4 DH Rule Addition (does not replace rule): The designated hitter
and the starting pitcher may be the same person. If the pitcher opts to bat for
himself, he is treated as two separate positions - a pitcher and a designated
hitter (abbreviated P/DH on the lineup card) - and may be substituted for as
such (i.e. if he is removed as the pitcher, then he may remain as the designated
hitter and vice versa). However, if a player who starts the game as a P/DH is
relieved as the starting pitcher, he may not return to the mound even if he
remains in the game as DH, and he may not play any other defensive position
after being relieved as the pitcher.
on page 42 of: