AFTER PUSHBACK, WITH ALL ENGINES OPERATING AND THE AIRPLANE BRAKES APPLIED, THE GROUND CREW REPORTED DIFFICULTY DISCONNECTING THE TOW BAR FROM THE AIRPLANE. THE FLIGHTCREW RELEASED THE BRAKES AT THE REQUEST OF THE GROUND CREW, AND THE TOW BAR WAS DISCONNECTED FIRST FROM THE AIRPLANE THEN THE TUG. WHILE THE GROUND CREWMEMBER WAS RECONNECTING THE TOW BAR TO THE TUG, HE OBSERVED THE AIRPLANE MOVING FORWARD AND ADVISED THE CAPTAIN TO APPLY THE AIRPLANE BRAKES. THE AIRPLANE THEN COLLIDED WITH THE TUG.