EMPLOYEE WAS ASSISTING IN HOOKING UP A SUPPLY TRAILER TO A VEHICLE FOR MOVEMENT TO ANOTHER LOCATION. THE TRAILER WAS EQUIPED WITH A MOUNTED JACKWHICH HAD NOT BEEN USED WHEN THE TRAILER WAS DROPPED AT THIS LOCATION. IT COULD NOT BE USED AT THIS POINT AND THERE WAS NOT ENOUGH CLEARANCE TO POSITION IT. JACK HANDLE FLIPPED UP, STRIKING EMPLOYEE IN THE MOUTH.