The “NOT” moving

 In Classical Tai Chi, the "NOT" moving at the junction of Yin and Yang represents the essential principle of Internal Discipline. The "junction" is a specific area in the torso or joints—such as the waist or upper-quarter body—where the stationary "Yin" (supporting structure) connects with the active "Yang" (moving part). 

This concept can be understood through the following key points:

• Fast Relaxation/Energization: The "not moving" (stillness) at this junction is not passive. It requires rapid relaxation—called Song (松)—to release tension and "energizing" to maintain the structure at the same speed or faster than the moving Yang part.
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• The Goal is Internal Power: When the junction is held still and relaxed, it allows the internal energy (Qi) to compress and flow across that point, creating a "whirlpool" or spring-like effect. This builds immense internal energy in a relaxed body, which can be instantly released.
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• Preventing "Double Weighting": It is the "pivot point" where one side of the body acts as a support, allowing the other to move freely. Your body will not be “double weighted”, aka “double heavy”. Being still at junctions will enable this, and your balance will be maintained. 
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• Distinction Between Yin and Yang: Master Stephen Hwa emphasizes that to distinguish between stillness and motion truly, the Yin part must not move at all, while the Yang moves. As Yang is "hidden" within the stillness of Yin, the contrast between them is heightened, which is key to developing Fajin (the release of power).
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• How it feels: When performed correctly, the practitioner feels a stretching sensation and intense energy flowing across the junction in an otherwise relaxed body. 

In summary, this concept means the junction should not move externally, but must remain dynamically alive and "ready" through instantaneous relaxation and structural integrity. 

Classical Wu Style Tai Chi application of Hand Strums the Lute 手揮琵琶

Application of Hand Strums the Lute 手揮琵琶

"If the opponent raises up, I seem taller;
if he sinks down, then I seem lower;
advancing, he finds the distance seems incredibly long;
retreating, the distance seems exasperatingly short."

Excerpt from: THE TREATISE ON T'AI CHI CH'UAN
Attributed to Wang Tsung-yueh
Researched by Lee Scheele

This application is demonstrated by Master Stephen Hwa here: Hand Strums the Lute

This describes the dynamic internal energy for the variation on Hand Strums the Lute 手揮琵琶
This variation of "hand strums the lute" occurs in the Classical Wu Style Tai Chi form a number of times (3) that precede and intersperse the repetitious movement in Parting the Wild Horse’s Mane 野馬分鬃.
The internal discipline of the movement itself comes from the core with the arms being largely immobile.  The lifting of the opponent comes from what might be called a lifting movement of the core at the waist.  An analogy might be described as opening the lid of a trunk with all the energy at the hinge. The body "opens" from a low to a high position wherein the student is inclining slightly back. One sees this type of vertical core movement "opening" in the "preparation" movement, the first movement of the form albeit without inclining.

Master Hwa:  "In the Jou Tsung Hwa's gathering I picked a guy to come at me.  He came at me, I sat back and threw him. Everybody (laughter) and I picked a big guy, so he came at me. So you see, let me do it slowly like in the form. It is really this move that is before "parting". This is a movement to handle people who come over the top at you and you do this.

Student: (laughter) that was hard for me to either lower myself or fall over.

Master Hwa;  You notice when a bigger person comes at me, I do not raise my hands.  My hands are still down here, I just lean the body back. With my arms down here they are very strong, if I raise them then I cannot lift him.

Student: All I felt was the integrity of your form, I did not feel any pressure.

Master Hwa:  Right. When I did it at the Jou Tsung Hwa festival, a guy that I threw, said "you do it very smoothly".  You do not feel any kind of forced movement. Very natural, I think.  This is why we never lift our hand over our head in the form or otherwise. If someone comes over the top I do this.  This looks like the hand is over the head but it is not.

Correct Stepping in Cloud Hands

Correct Stepping in Cloud Hands and "Yin-Yang Pairs"

(video link)

"The way leads from the sacrum upward in a backward-flowing way to the summit of the Creative and on through the house of the Creative; then it sinks through the  two stories in a direct downward-flowing way into the solar plexus and warms it. Therefore it is said: "Wandering in heaven one eats the spirit-energy of the receptive".  

Excerpted from : "The Secret of the Golden Flower: A Chinese Book of Life", Richard Wilhelm, Translator, Translated from German, Cary F. Baynes,  C.G. Jung, Commentary, see p. 61 for the passage.  Richard Wilhelm, C.G. Jung and Cary F. Baynes also being collaborators on the Wilhelm/Baynes edition of the I Ching or Book of Changes.

In the context of "secrets", I see that there is no shortage of books being published that tout learning "...the secrets of Tai Chi..." in some cases they claim that the "secrets of the masters" are to be found in the "Tai Chi Classics".  Certainly we know that there have been high level masters of Tai Chi in the early days but as Master Stephen Hwa states: "These Classics started becoming known in the 1930-40s when books about Tai Chi first started to be published in China. These writings reported in those books are a collection of short works attributed to different authors from Chang San-Feng to anonymous authors. Even though their authorship cannot be verified, they are considered to be the holy writ on Tai Chi. They do contain some insights and principles of Tai Chi; but also some glaring mistakes."

We know now however, that regardless of any supposed or real literary accomplishments, the early masters structured the Tai Chi form to serve two purposes:  It was designed to be useful for martial applications and also to be good for health purposes.  One has to admire the thinking behind such structure and I think it can be safely said that it was designed by people with sound reasoning skills.  Reasoning however that was no doubt based in whole but certainly in major part of their own martial art experience.  Thus we have the 2 major components of the "logical structure" of Tai Chi but they built it with an understanding that it need be composed of many facets or building blocks for the structure. An example of only one building block  but a major one is how "...every move in Tai Chi needs a moving part and a stationary part (yang and yin)..."  Those "parts" come together at junctions in the body, some can be in the torso, some can be outside of the torso.  These junctions can be said to be the repositories of the two parts or what are "yin-yang pairs" but regardless of what they are called, their location whether inside or outside of the torso is what is of major importance.

When inside or "internal" to the torso the yin-yang pairs will have the most profound effect on the "two purposes", when outside the torso, it is simply an "external" movement and cannot provide such enhancement to the internal system of the body. There are so many movements in the Tai Chi form however, can there be one that provides an examplary illustration of what we say here?  I think if there is to be any outstanding example it must be the "Cloud Hands" movement of the Tai Chi form. I have included this video link which specifically shows Master Hwa not only demonstrating the movement but discussing how to do it correctly:  

"Correct Stepping in Cloud Hands with application"

"Correct Stepping in Cloud Hands showing pullback"

I will let Master Hwa do the rest because I can certainly explain it no better but will conclude by saying IMHO that the movement you see in his back is the characteristic "ripple effect".  In this video you see the force, an "intense muscle undulating motion" .  This is a premier example of the saying about Tai Chi that it is "outside cotton but inside steel" . In my humble opinion or at least conjecture,  I see parallels in the up and down motion of his back or "undulating motion" and how this is described in so many writings about Taoist meditation (see the Golden Flower comment). Or parallels as to how the "chi" is "tapped" along the spine, travels to the extremeties, then moves back to the body core for further use. 

 On the other hand, although I think the Secret of the Golden Flower is written in such an ethereal style (it has been long associated with "hippy-dippy, flower style Tai Chi of the 60's in the U.S.") that it is almost unintelligible to me in English, there is nothing ethereal about what we see demonstrated by Master Hwa, After all, and as we correct our stepping in cloud hands, it is  as Master Hwa says in Uncovering the Treasure: "...a bad posture will stop internal energy generation and qi flow. No mental state or wishful thinking, (etheric or otherwise) can overcome that".