Each project milestone has it's own escrow payment. Before the work on that milestone is started, escrow is funded by you. When work is done, you release that escrow once you verify the work is satisfactory and complete.
If the work is not done to your satisfaction or accordingly to the milestone agreement, then you do not have to release that milestone. The messages between you and freelancer are all recorded for dispute purposes.
After you release an Escrow payment, we take a 10% fee from the freelancer's portion. It's just a small fee that freelancers pay for us finding them paying work. We only get paid when projects are successful so it keeps us honest.