Audi,
Excellent post. As usual I'm going to have to work through some of the things you talk about before I can speak clearly about them, but also as always you have given me much to think about.
I did want to mention though that my concept of the "spinal bow" comes directly from the spine, but it doesn't visually or even mentally look much like a classically drawn bow string, rather I think of it as a place to store energy to be used in conjunction with the other body bows, which due to the joints of the arms and legs do physically resemble a pulled or released bow string.
The way I see it now, without one bow the other two don't really mean very much.
If my legs aren't storing and releasing energy, then the spinal bow won't make too much difference. If the body isn't bowing, then the arms bowing won't do very much. They have to work together.
It all starts with the legs, the power gets stored then released in the leg bow, then the proper positioning/use of the waist/hips/kua and the release of the energy stored in the spine allows that energy to join up and combine with the energy from the legs, they become one and travel on to the arms, proper placement of the shoulders and arms allows correct transmission of this energy to the arms, then the arm bow accepts this energy, adds its own to it, and then allows all three energies, combined together into one, to continue on in one integrated, greater whole to be released by the fingers.
All that said, if you're not doing even one of them correctly, then they will not combine and you won't have all that energy combined at the point of contact. It's the combination of all three stores of energy into one, concerted force, unified and combined to be greater than the sum of all three that makes Tai Chi martial power so devestating as I understand it.
Now you can, quite obviously, generate quite a bit of power using only one or two of these stores of energy. This can be shown by Shoulder Stroke, where the arms aren't really involved very much. However, I still feel you will get a "greater whole" if your arms are in their proper positions. You won't really contribute their stored energy to the mix, but if they aren't positioned to allow your shoulder to be in the correct alignment for the strike, then you will still compromise the whole body energy of the strike. So the arms still play their part in this strike even though they may not add too much kinetic energy to it.
Think about the transition from Raise Hands to White Crane and the Shoulder Stroke inherent in the movement. Isn't it still required to have the arms rounded and extended as you make this stroke? It is every time I've been taught the movement. The reason as I see it is that the proper positioning of the arms here will lend a certain "stabilizing energy" component to the rest of your body to help integrate all the energy from the legs and spine.
I hope that all makes sense?
If it does, please explain it to me. I've confuzzed myself again...
