I'll post a letter from my bank from a few months back regarding the fund holds:
Merchants requesting authorization for purchases = automatic in most online processes.
Dear [cvxfreak],
When you make a purchase using the MasterCard debit feature, the merchant will request authorization for the purchase. This authorization will place a hold on your account for up to four business days. The merchant will then submit a settlement transaction to claim the funds for the purchase. If the merchant duplicated an authorization, they will usually correct the error by submitting only one settlement transaction. The duplicate authorization hold will be released within four business days. Merchants generally submit the settlement transaction by the second business day after the purchase. If the settlement transaction is not submitted within the four business days, the hold will be released and the balance on the account will increase by the amount of the hold.
Merchants requesting authorization for purchases = automatic in most online processes.