How does what work?  Fractional reserve or auditing them with block explorer?

If you mean, how does one audit your addresses with block explorer, simply check the addresses that you have to make sure that there have been no transactions that you didn't do yourself and that the address balances add up to what you should have.  If you have more than what you should have, then the website is sharing accounts among users and tracking the individual values internally.  Which would allow the website owner to loan out portions of the pooled balance, much like how fractional reserve banks do now, since they don't actually keep your money on hand.  If there has been any transactions in or out of said accounts that you didn't perform, then that means the website owner is using member accounts to "float" his own activities.

EDIT:  Except I can't actually see all of the addresses used in my Mybitcoin.com account.  That's going to be something I need to ask for.