When you put a credit card on file for room charges, your key is charged when:
1) You hit your room limit before the end of your stay. This has happened to me.
2) At the end of your stay and you haven't hit your room charge limit.
During my August Polynesian stay, my card was charged when my room charge limit reached $1500 after four days (I charged park tickets and shopping treasures from, 4 kids). After the charge went through my room charge was reset back to $1500 and my card was charged for $300 additional stuff when we checked out.
The room charge limit varies with the level of resort. Value resorts have a $500 room charge (pretty useless for me), Moderates have a $1000 room charge limit (better), and deluxes have $1500 limit.