A B757 ROLLED BACKWARD SHORTLY AFTER ARRIVING AT THE GATE AND AFTER THE CAPT HAD RELEASED THE BRAKES. THE CAPT RPTS THAT HE THOUGHT THAT HE HAD RECEIVED THE CORRECT CHOCKS- IN SIGNAL FROM THE MECH; BUT IN HINDSIGHT REALIZES THAT HE THOUGHT AT THE TIME THAT THIS WAS UNUSUAL. NO INJURIES OR DAMAGE RESULTED.

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PARKED AT GATE IN CLT; CALLED FOR PARKING AND SECURING CHKLIST; FIRST ITEM IS PARKING BRAKE. I RELEASED BRAKES; THE ACFT STARTED TO ROLL BACK. FO ESTIMATED ROLL BACK 6- 12 INCHES BEFORE I RESET PARKING BRAKE. I WAS QUITE SURE I RECEIVED CHOCKS-IN SIGNAL FROM SIGNALMAN. THERE WAS NO DAMAGE OR CONTACT AS ACFT WAS STILL CLR. THE MECH WHO WAS TO CHOCK THE ACFT WAS NOT IN PLACE AND DIDN'T GET THE CHOCKS IN UNTIL AFTER I RESET THE BRAKES. I USUALLY LEAVE THE BRAKES SET FOR A SHORT WHILE BEFORE RELEASING THE BRAKES. MOST OF THE TIME THE CHOCKS IN PLACE SIGNAL IS BOTH THUMBS POINTED IN; ARMS AND HANDS MOVED INWARD AT HEAD LEVEL. THE CORRECT SIGNAL IS BELOW THE WAIST. THE SIGNALMAN GAVE THE CORRECT SIGNAL WHICH IS UNUSUAL. ABOUT THE ONLY THING THAT WILL PREVENT ROLL BACKS IS TO HAVE HEAD OUTSIDE WHEN RELEASING THE BRAKES AND WATCHING VERY CLOSE (EVEN WHEN YOU ARE SURE YOU HAVE BEEN GIVEN A CHOCKS IN SIGNAL). I DIDN'T LEAVE THE BRAKES SET FOR THE WIND BECAUSE IT WAS OUT OF THE W AND WE WERE ON THE E SIDE OF BUILDING OUT OF THE WIND I THOUGHT; BUT IT PROBABLY DID AFFECT THE TAIL.