The second definition in my dictionary is more helpful, “fake;” interestingly, the fourth definition is “virtual.”
The radical for the character xu, is hu (tiger). When a tiger stalks, he forgets his body, he thinks only of the prey. Xu is the character used by Chinese Medicine in the expression shenxu (kidney depletion). When we go without food or sleep our bodies often become deficient and depleted, we lose fine motor control, the ability to focus, and concern for the flesh.
In the context of internal martial arts, xu is the fruition of the whole body moving as a single liquid unit. Xu is a description of the physicality of an “I can sense what you are doing, you can not sense what I am doing” situation. A body which is xu is unstoppable because it doesn’t apparently respond to resistance.
I know what you are thinking, zombies are xu. That’s right, if zombies could talk they would be like, “Yo, I don’t care if you chop off my arm, I’ll still eat you. Shoot off my leg, no problem, I’m still coming...” I hesitate to say that xu is a form of disassociation because it is not necessarily a psychological problem. However, the first time I bang my body or my leg against the ground teaching daoyin, people wince. They think, “Are you crazy?”
Xu is external martial conditioning. Xu is the result of pounding and slapping the outside of ones body as a way to be comfortable with heavy contact.
It is also what allows self-mortifiers to pierce and pummel themselves. There is a long history in China of using a ritual trance initiation to induce xu. Often it involves a ritual emptying, as in nuo theatrical exorcism where the hun and pö spirits are removed from the performer’s body and placed in jars using talisman and mantras. But it is also a quick way of training troops. During the Boxer Rebellion (1900) each boxer went through an initiation process which made him immune to pain and of course (he believed) bullets.
In trance the mind is totally preoccupied. The boxers would invoke their personal deity and they would become, for instance, the Monkey King. By preoccupying the mind with all the attributes of the Monkey King the individual boxer must have been able to disassociate from any injury to his own body. He may also have been hungry and been entranced by the idea that he was purifying the country of evil Christians.
Other examples of training troops quickly involve group chanting. Qawwali music from Pakistan, for instance, is all about invoking love. It is the idea that while you are butchering your enemy you feel intense love for them, as you send them to god, you also make them one with god. Because you are so focused on love, you disassociate from your own body. Intense anger, revenge, and envy work too. As Laozi says, “When we are possessed by desire, we experience only the yearned for manifest.”
Many spiritual traditions think of xu as a form of transcendence. Putting on my rational 20th Century hat, I’d say that xu is the result of two forces; hormones (probably adrenaline, dopamine, oxytocin, epinephrine) and mental focus.
(While mentally focusing on an idea, a goal, or an object outside the body can create an experience of xu, "focus" is a really bad word choice because the more spatially expansive (capacious) ones awareness is, the more xu the body can become.)
For those who practice internal martial arts xu comes about simply through relaxation. In fact I would tentatively say xu is relaxation. When every sand sized particle that makes up your entire body is relaxed it is xu. (Xu is used in the Chinese character for atom.) A body which is xu does not intentionally respond to resistance. It is heavy, liquid and unified. Actually it does respond to resistance, but it does so in an unconditioned, unconscious, uncontrolled automatic way.
Everywhere I look these days people are abusing the poor word “embodied.” Everything needs to be “embodied” these days, if you want to sell it--it better be embodied with some awesomeness. Exercise, politics, education, shampoo, coffee, even the truth is supposed to be embodied. But I’m telling you people, if you take this ride to the top of the hill, it ends with a totally disembodied experience. But words are misleading, truly internal martial xu should be both embodied and disembodied at the same time. When all the controlling, micro-structural, 'I own this body,' 'this is me,' 'this is me-ness,' voices get turned off what is left is xu. Xu and emptiness (kong), of course.
I’m not exactly describing an ego-free experience here. The ego just becomes bigger, it lifts off of the body and becomes spacial. One experiences a lively, dynamic form of perceptual-motor spacial awareness.
Everyone is at least a little bit xu all the time. And everyone is capable of getting really xu in short order. Most of the drugs you can name off of the top of your head increase ones experience of xu.
What inhibits the experience of xu? Only one thing: Feeling in possession of your own body--believing that what defines you is limited to this empty bag of flesh.