EE WAS ASSISTING DRILL OPERATOR CHANGE HAMMERS ON DRILL. TOOLS USED WERE THE HYDRAULIC OPERATED PIPE WRENCH AND A J WRENCH. EE PLACED J WRENCH ON THE DRILL STEEL SO THAT IT WOULD REST AGAINST THE MAST WHILE THE PIPE WRENCH TURNED THE HAMMERWHEN HAMMER BROKE LOOSE, THE J WRENCH KICKED BACK STRIKING THE EE IN THE RIBS.