everything wrote: Second the hips must open and close appropriately to let the legs express yin and yang.
anyone care to elaborate on the hip sections from above?
He stated,
The sensitivity that Wang cultivates enables him to read the opponent’s centre and the depth of root they have in their feet. Master Ma and Wang had very deep roots as opposed to many whose roots barely penetrate the surface. Those many may relax and sink but they do not use the yi and qi to nail down into the earth. To do this it is necessary to sink the qi and loosen the hips so that they become alive, strong and stable. The problem for many is the hips are not stable, or if stable, not loose and alive. It is this sinking of the qi together with the establishment of zhong ding that Wang uses to repel an opponent. He remains soft to sense the opponent’s jin and root, then he uses the yi (intention) and qi to activate the jin which issues from the zhong ding, and throws the opponent out. Wang avoids using his arm jin which the opponent could lock onto and use back against him. Using jin in the arm would de-stabilize zhong ding and allow an opponent to lock his arm and throw him.
Wang said: "There are 3 ways to make the dantian low so the zhong ding becomes very strong. First do the Taiji form and basics, always letting the dantian sink. Relax and sink the body. Second the hips must open and close appropriately to let the legs express yin and yang. Then the energy from the dantian can drop down.
The difficulty, for many, is to de-code or translate the traditional verbiage into identifiable physical actions and their mental components. Having a skilled teacher who can show you how to do what is being described by the language is essential, in my opinion.
He states that they "had very deep roots...to nail down into the Earth". This isn't literal. Human don't have roots (like a tree) and their feet, generally, do not extend into the ground: humans stand on top of the surface supporting them. Humans can push down on the ground and the ground can push back with an equal and opposite force. Humans can resist sliding on the surface of the ground based upon the friction between one's feet/shoes and the surface on which one stands. There's no magic and no mysticism there.
When someone says they "had very deep roots" that penetrate into the Earth/ground, what are they actually talking about? From the opponent's perspective, he isn't able push, topple or slide the practitioner. From the practitioner's point of view one is neutralizing incoming forces, the result of which is that one isn't pushed, toppled or slid. The relevant question is then how is the practitioner doing that?
Traditional language states that the practitioner is able to do that when he, "remains soft to sense the opponent’s jin and root, then he uses the yi (intention) and qi to activate the jin which issues from the zhong ding, and throws the opponent out" and "make the dantian low so the zhong ding becomes very strong... [by] doing the Taiji form and basics, always letting the dantian sink...and... the hips must open and close appropriately to let the legs express yin and yang". there's a whole lot in that statement that needs de-coding into practical action/training, as follows:
1. Softness is used to sense an opponent's jin and root.
2. intention and qi activate jin
3. jin issues from zhong ding (central equilibrium)
4. zhong ding is strengthened by "lowering the dan tian" (or lowering qi to the dan tian)
5. doing forms and basics, while always letting the dan tian sink is training for the above
6. opening and closing of hips "lets" the leg express "yin" and "yang"
7. expressing yin and yang in the legs helps sink the dan tian, which strengthens zhong ding.
So, you have asked about a small piece of that, specifically, opening and closing the hips to allow the legs to express yin and yang. A short answer is that different styles of Taijiquan use the hips differently. However, each allows the body to create a conduit to the feet from the point of application of a force. The hips must facilitate that conduit, rather than put a kink in it, impeding transmission of force to the ground and transmission of force from the feet on the ground to the point of application. The hips are one link in that chain, or one section of the hose/conduit. To effectively transmit, the entire conduit needs to allow unrestricted transmission. The practice of forms and individual exercises are, in large part, about establishing that conduit. Different styles of Taijiquan have different approaches to training that. The actions - mental and physical - that produce that ability are often referred to as "letting the dan tian sink" or "sinking qi" and is related to "rooting".