You would wrap a tree trunk with ropes, and keep punching it. You throw 5000 punches day and night — do that for a month, the inside of your fist swells up until you can barely curl your fingers. Then you open a tin can and set it up on a stand. You keep punching the sharp part. When your hand turns into mush with blood and pus, you start punching a pile of salt. Repeat it, and your hands become like a stone.
L’entraînement des forces spéciales en Corée du Nord.