When you use  a debit card it has to very useful features. A debit feature in which you use your pin number with and the account is then debited right away. And a credit feature in which a authorization hold is placed on the account, telling the banking institution that you paid for something with your card, and a sales draft will follow. 

Lets use our imaginations

You have 100.00

You are a avid debit user ( never use credit) and you use a transaction register.

You spend 20.00 at Walmart with debit  - your new balance is 80.00

You spend 50.00 at the salon with debit - your new balance is 30.00

When you use debit, you can call in to the automated system, sign into you online banking, and see these as pending transactions and deducted from you balance.

Now, still using our imaginations

You have 100.00

You sometimes use debit and sometimes use credit, and sometime use a transaction register

You spend 20.00 at Walmart ( Debit ) - New Balance 80.00

You spend 50.00 at the salon ( Debit ) - New Balance 30.00

You pay your water bill over the phone 30.00 ( credit ) - New balance 0.00

 When you pay something over the phone, on the internet, or not using you PIN number, that is a credit transaction. Everytime you swipe your card, your bank received information about that  transaction. When you swipe the card as a credit  it places a hold on the account for the purchase. Therefore, pulling from your balance. Just like when you use your card as a debit. However, a credit transaction takes 1 - 3 business days to clear, and still can hold the account for the purchase every day, until the bank receives a sales draft. Therefore, you will not see the transaction pending until the day the bank recieves the sales draft. 

You should pull credit transactions from the balance as soon as you swipe that card, give out the number, etc. Just like you should when you write a check. It hasn't come in yet, but it will.  

Now let's say, you go home and check your balance. You see all of your debit transactions pending, but not the water bill payment......  

You go out later that night, and Uh oh ... your car is out of gas.  You use your card at the gas station for 25.00 as a debit to get gas. You just overdrawn your account by 25.00 because of the hold that is on the account for the water bill. 

I assure you that if you use your cards as debit as much as humanly possible, you cannot overdraw your account.