When you use your debit card, the PIN is entered and the funds are immediately deducted from your account.
When you use your debit card as credit, you may sign for the transaction instead of entering your PIN. The transaction is processed and the funds are placed on hold in your account. The transaction is usually settled within a few days.
Whether you use your debit card to make a debit or credit transaction, the result is effectively the same: The money is withdrawn or held in your account.
Debit or Credit transactions will only process up to amount of available funds in the account.