A regular debit account does not post debits and credits instantaneously, which is why there is such a thing as overdraft insurance for a normal debit card.
Overdraft protection just means they automatically transfer funds to cover, and don't penalize you as they otherwise would. It does not mean that your account cannot be over-drafted.
It would not be within sharia to spend money without that money being on hand-hence, a debit card that does instant debits/credits would comply with the rule, and that's why it's being marketed to those who desire shariah compliant products.
A shariah compliant account means your savings funds are complying with a particularly large body of Islamic law dealing with loan transactions, interest, and contracts. That's a whole lot more than a simple debit/credit issue. It would not be within the shariah laws that have developed around banking to just have a normal account that garners interest or that charges interest.
Edit: Here's an article that explains more about what Islamic banking would entail, such that one would want a different account in order to comply:
http://www.metimes.com/Business/2008/04/24/islamic_finance/6986/