It’s as if their body was already dead and they had no attachment to it. The name for this quality in Chinese is Xu. Wang Xiangzai said, Xu Kong Ling Tong: Body as if dead (Xu), emptiness inside (Kong), lively elasticity of the spacial mind outside (Ling), and body functioning as a single liquid mass (Tong). So simply having Xu would not be enough, but it would be a heck of a head start.
It’s my theory that some of the founders of great martial arts systems were tortured or had torturous experiences which showed them that they could separate their spacial mind from their physical body in such a way that pain had no effect. And by no effect I mean that they didn’t feel the need to contract, recoil, or tense up in response to it.
And that brings us to Systema. I haven’t written about systema, or seen much in person or played around with anyone who swore by it, and I keep forgetting to order the book. But it has a lot of devotees and you can watch hours of Systema videos on Youtube.
The founders of Systema were members of the Russian special forces. From first hand accounts I’ve heard, hazing is a constant in the Russian military. So it wouldn't be too surprising to learn that the founders of Systema were tortured at some point. The Russian Orthodox Church kind of has a history of that too. It is my suspicion that they were among those rare individuals who happen to find torture liberating, in the sense that it freed them from fear of body inflicted pain.
Systema training has a lot of different types of hitting and beating, with small sticks, with big sticks, various objects and with hands of course. They have a whole thing about how you have to release the fear with the breath.
We could posit that there is a martial arts history of torturing people to perfection.