Hand training is a method used by martial artists as a means of strengthening the palm and side of the hands. The method is simple and very painful—one just hits something hard (such as a block of wood or a sand bag) over and over for a set period of time each day. Calluses will form after some time. Eventually, after repeatedly breaking the calluses and letting them heal, the hand will be very hard in the trained spot.

Most martial artists don't recommend this method. Aside from the extreme pain it involves, it is irreversible and leaves the trained area disfigured and black.