When you use a credit card to enable room charging, the card is swiped when you check in - but not charged. Each resort has a "credit limit"; Values are $500, Moderates are $1000, and Deluxes are $1500. When your rom charges reach that limit, your credit card is charged, and the limit is reset. Some folks have reported that they were called down to the front desk when this happens and had to authorize resetting the room key, while others simply keep charging with no interruption.
I think the reason for this is so that Disney can limit the number of individual credit card transactions they have to process; instead of processing your purchases on a daily basis, they process them based on the dollar amount - every $500, or every $1000, or every $1500. It makes perfect sense to me, saves Disney some money, and is transparent to the Guest.
However, you also have another choice - you can put down cash, Disney Dollars, or a Disney Gift Card to enable room charging. If you put down $500, and only spend $450, you'd get the leftover $50 back when you check out, in cash.
NOTE: Not all check-in CMs know how to take cash/DD$/Gift Cards to enable room charging. Some will even say, "No, you can't do that, it needs to be a credit card." But it is possible, I have done it a half-dozen times; if your CM doesn't know how, ask him to call his supervisor.
