INJURED WAS HOLDING A TORQUE MULTIPLIER UNDER A DOZER BLADE ON A NUT, WHILE THE MECHANIC APPLIED THE RATCHET ON THE INPUT SIDE. THE HANDLE OF THE MULTIPLIER WAS BRACED AGAINST A STEEL RIB UNDE R THE BLADE. THE HANDLE SLIPPED PFF THE RIB CATCHING THE INJURED'S THUMB BETWEEN THE HANDLE AND THE DOZER BLADE.