What is happening? They are both leaning in using a moving A-frame structure. Confucius said, "If I show the student one corner of the square and he does not show me the other three, I change the subject." What are the other three? The A-frame is the most common type of counterbalance. If one of them were to suddenly disappear, the other would stumble forward. The reason people use this A-frame so often is that it feels powerful, it creates the illusion of power. But it is not powerful. To be even marginally powerful one would have to get a 45 degree angle from the arm contact-point to the ground. But it feels powerful. The other three corners of the square do not have this illusion, they feel weak, so most people do not train them.
Yao Yue, has inward strength (neigong), his partner does not. Yao's partner has superb structure and continuous rooting.
My improvisation theater teacher Keith Johnstone explained that initially he had no idea what he was doing but was able to invent improvisational teaching games by reversing everything his teachers taught him. That was his "method."
In the famous Neijia quan (Internal Martial Arts) epitaph for Wang Zhengnan (1676), Huang Baijia writes, "Shaolin is the peak of refinement for the external arts. Zhang Sanfeng was a Shaolin expert, but he turned the art on its head and thereby created the internal school. Obtaining just a little bit of it is enough to defeat Shaolin."--from Brennan Translations
Meir Shahar translates this, "..having mastered Shaolin, he reversed its principles..." -P. 176 (2008)
I would like to see more people try their hand at this. I do not claim to have a better understanding of the language than either of them, but I'm temped to translate it, "..Having perfected Shaolin Jing 精, he flipped it..."
In any event the meanings are similar to Johnstone's idea of reversing what his teachers taught him. That is how I see internal martial arts, everything I learned was wrong. I have only gotten good by reversing nearly everything I was taught.
In the video, Yao demonstrates how utterly vulnerable rooting with good structure is to an inward strength (neigong) attack. Yao's partner is trying to keep his shoulders down throughout the cooperative exercise, that is good for his structure but not for whole-body-liquid unity. Because he loses unity, he has to bring Yao's force down through his body to the ground--a huge loss of time and effort. If you watch closely, he looks uncomfortable when he is on the inside of Yao's arms and he is more comfortable when he is on the outside. Yao doesn't care because the potential power does not change for him.
Yao shows something about his back leg, which I would immediately try to reverse since it looks wrong to me. Then he does a push which is not a push, it is just forward momentum without losing whole-body unity. But most people will see a push. Then he shows a couple of punches and a potential head butt. Fun. Easy to do the Shaolin way, but this is internal martial arts so you have to reverse them. Or turn them upside down. Or figure out what the other three corners of the square are.
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