It is a simple matter of accumulated experience, of having been hit or stomped often enough to forget the ugly panic that nice people associate with a serious fight. A man who has had his nose smashed three times in brawls will risk it again with hardly a thought. No amount of instruction in any lethal art can teach this—not unless the instructor is a sadist, and even then it would be difficult because the student’s experience would be artificially warped and limited.
