If you hand your debit card to a waiter at a restaurant and they take it back their cash register to charge your debit card they'll swipe your debit card and select the option to charge the debit card as credit card.
For a credit card transaction no PIN number is required for the transaction so the payment will go through without needing to enter the PIN number.
They simply just charge the debit card as a credit card so they don't need the PIN number.