On/Off - Empty/Full
/Every Taijiquan enthusiast is familiar with the phrase Taiji comes from Wuji. Wuji being emptiness. But there are all sorts of related ideas like formlessness, or without existence, or without substance, or non-being.
It's common for a teacher to hit you lightly and say, "You're empty here, wake up!"Â Â Likewise, a teacher may hit you and say the opposite, "You're too full here, that makes you easy to control."Â Sometimes a posture is defined by having one leg empty and one leg full.
When people first try to fight without training, they usually make themselves full in the upper front part of their body. They come at you with both arms. This kind of attacker is very easy to control by turning, going around their defenses, or by kicking. As a person develops skill he will be full in one place and empty in another. The emptiness draws an opponent in so that one's fullness can blindside them. If I deliver a committed punch or kick to my opponent and that strike completely misses, It's likely that I've just made myself really vulnerable.
Nearly all experienced fighters, whether internal or external, boxers, wrestlers, or judoka, use a combination of empty and full. Brazilian Capoeira has it too, they call empty and full, 'the honey and the poison.'
Chapter 11 of the Daodejing is about empty/full. I'm really hesitant to put up a translation because the terms it uses are problematic.   It's basically a simple chapter that first describes a wheel with 30 spokes, then a clay vessel, then a house with doors and windows. The standard translations effectively put a comma in between the word "not/empty" (wu) and the word "have/full" (you)." (There are a number of translations that allow you to follow along with the Chinese text, which is a really big help.  If you don't have a copy to follow along with, you can just trust me on this one.) Thus the chapter is said to mean:  The hub of a wheel, the space in the vessel, and the door and window holes in a house are all empty, and yet it is these empty places which give each object it's ability to function. Thus, it is emptiness which makes things useful.Â
It's cute, but come on, it's also too obvious. There are hints that this chapter is about something else altogether. 30 spokes could be a lunar month, a cycle of time. Clay vessels are used in religious processions and are filled with offerings, they are left in tombs. And a house without doors and windows, what's that? A tomb, right?
The two oldest commentaries on this chapter, Xiang'er and He Shanggong, both talk about Daoist cultivation allowing us to see wu/you. Perhaps wuyou is one word, meaning empty/full. Perhaps it refers to seeing time itself, seeing each thing as having a life cycle-inception, development, decay, death/compost.
No matter how good a fighter you are today, at some point in the not too distant future, someone will be better. The Olympics is basically a display of peaks, in which we all know the individuals competing were slower or weaker a few years back, and they'll be slower and weaker a few years hence.
But I digress.
Martial arts skills are for the most part based on being able to quickly change between being empty and being full. Even the internal arts, like Taijiquan or Xingyi or Bagua, are essentially like boxing in this regard. Subtler yes, but in essence they are an "on/off switch." If you have an on/off switch, you are vulnerable. If my "on" hits your "on," the lights go out!
But perhaps it is possible to be wuyou, that is-- empty and full at the same time, with no on/off switch. That, by the way, is what George Xu is claiming he can do these days, and I think he's on to something!
I apologize if you thought I was going to have a definitive explanation of empty and full. I think they are provisional terms that get used a lot of different ways depending on the particulars of the teacher and the teaching.