This handy tool bolts on the bell housing in place of the starter. Inside you have a gear that engages with the flywheel ring gear, outside you have a large hex to put a wrench on to operate the tool. Now you can rotate the engine clockwise or counter-clockwise. This is useful to free a stuck engine, set timing, adjusting valves or to lock to remove/install the ratchet nut or to change a timing gear. U.S.A.