The activity in progress was the completion of a routine lifting activity that involved moving a steel deck plate with the fabrication shop overhead crane. The plate was positioned on the top of a prefabricated frame that was at waist height. The rigger proceeded to release one the lifting plate shoes, he then climbed onto the frame and crossed the deck plate to remove the second lifting plate shoe, on attempting the release of the second lifting plate shoe using the lifting webbing strop for leverage, the shoe came detached and struck the rigger on the side of the Head.