Different type of gung (skill), but like anything else in TCMA, chi is involved. It's a slow, gradual strengthening/toughening of the hands. A process that takes several weeks/months with a daily training ritual (not just striking a pad, but a ritual, from warmup to training to cooldown, with application of jow at certain points). Usually culminates in breaking bricks.

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"Pain can be a great teacher of compassion and humility."~ Unkotare-san

"Whatever the case, it proves that countless disasters can be prevented by simply assuming everyone you're working with is a moron." ~ Adam Brown, 5 Tiny Mistakes...