Maybe there is no hold at a gas station, but there is at other businesses.
I stayed at a hotel over the weekend. When I checked in the clerk said to me:
"Because you are using a debit card, we are going to put a hold on your account equal to the amount of the reservation. The debit won't go through until you check out."
When I checked my bank account later that day, the room charge was listed as "pending" and my available balance was that amount less than the actual balance.
Many car rental companies will also put a hold on funds when renting with a Debit Card. This site details the policies of some of the major rental agencies.
Just so that there is no misunderstanding as to what a "hold" is vs. a actual debit, take a look at the difference between Avis's policy and Enterprise's:
Avis:
"When using a debit card at Avis there is a minimum hold of the full amount including all taxes and surcharges of your rental. These funds will be frozen in your account; you will not have access to them during the period of your rental. The unused extra hold amount will be released at the end of the rental."
Enterprise:
At Enterprise "a minimum deposit plus the entire cost of the rental will be debited from the renter's account at the time of pickup."
Per that site, it could take up to 15 days for Enterprise to *credit* the funds back to the account as opposed to being *released* by Avis at the end of the rental.
There is a snack vending machine at my workplace where debit cards can be used. Up to 3 products can be purchased with one swipe. When I swipe my card, a hold is placed on my card in the amount of 3 x the price of the most expensive item. Overnight, the hold is released and the price of what I actually purchased is debited.