Once you put a credit / debit card on file, they only verify if it's valid. No holds are placed on the funds (other hotels do; Disney doesn't). Your credit limit will be based on the resort level: $500 at Values, $800 at Mods and $1000 at Deluxes, IIRC. If you reach your limit, they won't automatically charge the accumulated balance to the card; you have to ask them to do it. Only then will they charge the card and restore your CL. The only time they charge to your card automatically is on checkout. If your KTTW card is declined when making a purchase you can usually solve it with a phone call to the resort, telling them to charge the balance to the card. To avoid issues what I usually do is check our charges every couple of days and tell the CM to charge the card if I'm nearing my limit.
Two words of caution with debit cards, though (from personal experience

):
1) Check if your card has a daily purchase limit, or a maximum transaction amount limit. Call the bank / issuer. If it's too low, you can usually ask them to change it temporarily if you explain that you will be on vacation.
2) With some debit cards, when using it for tourist / vacation items like hotel charges, car rentals, etc, the issuer might debit the amount charged, and will also add a temporary hold on the transaction of about 15% of the amount charged, (for 'incidentals' is how they explained it to me

). The additional amount will be cleared in a couple of days, but it will affect your balance. This means that if you're clearing your balance at, say, a Value, Disney will charge $500, but your bank account transactions will show a charge of $575: the $500 charge plus the additional 15% hold. After a couple of days you'll see the charge revert to $500, with the $75 restored to your account.