DURING A "POWER-BACK" OPERATION FROM THE TERMINAL GATE, A GROUND SIGNAL PERSON AND TWO WING WALKER WERE USED IN ACCORDANCE WITH PROCEDURES. AFTER REVERSE THRUST WAS APPLIED AND THE AIRCRAFT WAS ROLLING BACKWARDS. THE SIGNAL PERSON COMMUNICATED AN EMERGENCY STOP SIGNAL TO THE PILOT WHEN HE SAW CROSSING GROUND TRAFFIC (ANOTHER AIRCRAFT). WHEN THE CAPTAIN SAW THE SIGNAL, HE TRANSITIONED BRAKES. AT THAT TIME, A FLIGHT ATTENDANT WAS THROWN AGAINST THE CORNER OF A CART IN THE GALLEY AND INJURED. ACCORDING TO THE POWER-BACK PROCEDURES, THE AIRCRAFT WILL BE STOPPED (BOTH IN NORMAL AND EMERGENCY SITUATIONS) BY A SIGNAL FOR STRAIGHT AHEAD MOVEMENT. THE CAPTAIN WILL STOP THE PLANE BY QUIETLY COMING OUT OF REVERSE AND APPLYING FORWARD THRUST. THE USE OF BRAKES IS PROHIBITED WHILE THE AIRCRAFT IS MOVING BACKWARDS. THE SIGNAL PERSON USED THE FORWARD SIGNAL, THEN CROSSED HIS WANDS FOR A NORMAL (BRAKING) STOP DURING THE EMERGENCY STOP.