An impactor handwheel has a crossarm that is directly coupled to the valve stem. The handwheel pushes against the crossarm to apply the torque. After the valve has been closed to a hand-tight position, the handwheel is backed off a bit (< 1/2 turn) but the crossarm stays in position (i.e. valve remains hand-tight). The handwheel is then spun quickly in the closing direction so that is 'impacts' the crossarm providing the much higher torque required to properly seat the valve. In large or high pressure valves, it may take several men to apply the torque required with a standard handwheel. With the impactor handwheel, the instantaneous torque applied when the handwheel strikes the crossarm can be as much 10 times greater (4000-5000 ft-lbf) than one person could apply with a standard handwheel.
Even though they can and are used on all types of vlaves our internal piping code restricts there use to globe valves and suggest they be limited to valve sizes of 8" and above.
If you go to this site and to page 65 of the pdf there is a good picture of hoe an impact hammer handwheel works.
Generally I wouldn't recommend a hammer blow handwheel on a gate valve. If you or "Big John the field guy" hammer it hard enough you can drive the wedge in so hard it won't come out or possibly even crack a casting.
