EE observing installation of catwalk from double drum hoist cab to the mezzanine. Crew was using overhead 15 ton hoist to place the catwalk. Became stuck. As the hoist continued to attempt to move the catwalk, it came free and caused catwalk to spring upward due to stored energy. Panel sprung upward and fell 15' to floor. Struck EE in both knees, puncturing L thigh, needed stitches