TWO MECHANICS AND A SUPERVISOR WERE ATTEMPTING TO RAISE THE TRIPPER BRIDGE TO MAKE AN UPGRADE. WHILE THE SUPERVISOR WAS ASSISTING THE TWO MECHANICS BY OPERATING THE MANUAL JACK, MOVEMENT IN THE JACK ARRANGEMENT OCCURRED CAUSING THE MATERIAL USED AS AN EXTENT ION BETWEEN THE JACK AND THE TRIPPER BRIDGE TO BE THROWN OUT. THE EXTENSION STEEL STRUCK THE SUPERVISOR IN THE HEAD.