When I did Karate... oh god my Sensei would thwap me for not knowing the name, but we practiced form with basically a wooden 2x4, vertically placed in a mount, with twine wrapped around the target area.

The point of the thing was repetitious form practice.

If you hit it too hard, it would recoil and then come back and hit you again.

Hit it too soft and there's no noise at all.

A neat way of moderating how hard you're supposed to be striking.

Anyway, sorry your hands go numb sometimes =(