It doesn't "turn" into AGP pro - there is no functional difference; AGP pro just has extra power pins for cards that need more juice. (This is the serious approach, gamer's cards have resorted to that silly "plug a power wire into the card" workaround).
The tab just is a mechanical blocker so you insert the normal AGP card into the exact right spot. Else you'd be able to offset the card - which is why the machine then doesn't boot.