Patrick wrote:But what is important is the mass behind it, no?
Important to what? What is the context of your statement?
For example, there are three ways that you can control your postural stability.
I'm not sure what the connection is that you are making between the current discussion, "The Dan Tian", and postural stability. Are you talking about in two-person work and fighting, methods of maintaining postural stability when forces are imposed upon you?
Simply, put, an object is stable when its center of mass is directly above its base of support. If it is not, the object will topple. Humans balance by maintaining their center of mass above their area of support. If standing, that is the area defined by their feet.
Walking, for example, is a series of controlled falls. We swing our center of mass beyond (in front of) our supporting foot and, before our face hits the floor, we swing the other foot out and land on it, creating a new support and stopping us from falling. In an adversarial situation, one strategy is, as the opponent is stepping, to sweep the foot he is about to land on. Since he has moved his center of mass beyond his base and you just prevented him from creating a new base of support under his mass, he falls.
You "sway" your whole body via the ankles (ankle strategy), you control your mass by folding at the hips (hip strategy) or you take a step. When you use the first strategy, the whole body is involved in controlling your mass. When you fold at the hips, the legs are not activley involved.
I'm assuming this is in the context of someone applying a force to you that will cause you to fall unless you respond ("change"). A section of my video is dedicated to exactly this subject. There are two primary strategies for keeping your center of mass over your base of support . The first has to do with the length of your base of support. If your base of support is very short in the direction of the applied force, you are easily toppled: a small displacement of your center of mass will make you unstable/fall. If you stand in a bow stance, for example, your stance is long, but narrow. If someone pushed in the direction of the length of your stance, you have a lot of "length" to work with. If someone pushed in the direction of the width of your stance, you have much less "length" to work with and are much more easily toppled. One option is to redirect the incoming force so that it more fully aligns with the long direction of your stance. This can be done is a variety of ways, including rotation of all or some of the body. Another option is to move your center of mass by shifting weight (center of mass) within your base of support.
The second strategy has to do with either changing the location or shape of your base of support. Typically, this is done by stepping or moving the feet.
When you fold at the hips, the legs absolutely are involved. The legs are connected to the pelvis by muscles and connective tissues: twist one end of those, at the pelvis, and it twists the whole thing.
When I look at Chen Yu, I can see that his "dan tian" movement entails a compensation movement. He slightly moves into the other direction to wind up. The power is located in the upper body and the legs are mere the basis for his upper body. His "dan tian" movement is simply a method to wind up and thus creating a longer way to accelerate.
You misunderstand the mechanisms involved. The power is not "located" in the upper body nor are the legs passively supporting his upper body.
In the context of what we are talking about, the use of the body can be crudely
modeled as a series of springs. A spring is a mechanical device that stores mechanical energy when it is deformed by the application of a force and releases that mechanical energy when the applied force is removed. There are three primary types of springs: axial (tension/compression), leaf and torsion.
An axial spring is, for example, a typical coiled spring. If you squeeze the length of the spring, the applied force shortens the spring, storing mechanical energy. When you stop applying the force, the spring returns to its original length, releasing the stored mechanical energy. This is like stretching a rubber band and then letting it go. The elasticity of the muscles and connective tissues of the body can be stretched and released. In doing so, they store and release mechanical energy.
A leaf spring is, for example, like the bow of an archery bow. Mechanical energy is stored when forces are applied that bend the spring. When the forces are removed, the spring straightens to its original shape and releases the stored mechanical energy. Traditional Taijiquan refers to five major bows: the two arms, the two legs and the spine. These are bent (or bowed) to store mechanical energy and straightened to release stored mechanical energy. The spinal "bow" is the most important.
A typical torsional spring is a clock spring. It stores mechanical energy by an applied force twisting the spring. It releases mechanical energy when the applied force is removed and the spring untwists back to its original shape. "Silk reeling" is the twisting and untwisting of the elastic parts of the body.
In many cases, movement, in Taijiquan, anyway, involves all three simultaneously and is what you are seeing in Chen Yu's student: twisting, stretching/contraction and bending.
I do not critique his movement, but my understanding for "one part moves, all part moves" is more along the line of the ankle strategy.
In my opinion, that is a misunderstanding.
Power is not created by winding something up, but by learning to drop your mass into your opponent and quickly stopping yourself from falling over.
As a kid, did you ever have a rubber-band-powered balsa wood airplane, the kind where you wind up a rubber band and let it go and it drives the propellor?
Dropping one's mass is one mechanism of creating force in martial arts. There are others.