Gift card / pre-paid cards and pre-authorization

Gift cards or pre-paid cards holds a fixed amount available for purchase or authorization.  Any time a transaction is authorized the authorized amount will hold the amount available on the card for additional charges.  If the authorized amount is more than the actual charge it may take a few days for the difference to become available again.

Good example video at http://www.giftcards.com/help/videos/FAQ_7.html