Think of it this way, your performance bond is held by oec during regular market hours. At the 3:15 pm closing the clearinghouse matches trades and determines open equitiy positions for all the clearing fcms. These fcm's then credit and debit respective customers balances and transfers or recieves net funds from the clearinghouse. All trades must be matched and settled at the end of the day per the clearinghouse. This is why even if you sell your position at 3:31 cst upon opening and you are under the performance bond requirements you will recieve a margin call and not be allowed to trade the next day or until you have adequete capital as that bond is posted against the previous day's trades.