Used in the training of WuShu to harden the hand's hit surfaces (knuckles, fingertips, palms, the carpals edges and others), and to enhance the grip's strength.
Students of WuShu are made to hit and grip the gravel with the many fist forms (dragon, tiger, crane and so on) hundreds of times per session, or at least until the gravel 'changes color'.
This form of training is often combined with iron fist exercises, in which the students hit hard surfaces, like walls, trees, heavy slabs of stone or metal, or with push-ups or handstands on the knuckles on small stones.
Related gripping exercises are, for example, push-ups and handstands on the fingertips.
A popular training tool used, among other things, to develop more of the body's hit surfaces, is the Wing Chun practitioners' characteristic wooden dummy.
think again before attempting to practice anything named here without an experienced teacher