I think an interesting perspective can be gained by contrasting the stances exemplified by Kuo Lien Ying and Wu/Hao usage. Kuo’s shin in his “bow stances” does not even reach the vertical and looks to my eye more like the (40-60?) stance that I think is extensively used in Chen style. As I understand it, he favored deep wide stances that are even larger than those shown in Jeff’s hyperlink to Li Yaxian.
I do not know much about Wu/Hao style, but I think that they tend to avoid entirely the “articulation point” implied by Bow Stances, by simply bringing the back foot up to the front foot in order to “end” a posture. I can imagine how all these variations could be effective in practice, but would insist that underlying each implies different practice methods. I would also say that within the Yangs’ forms, there are actually quite a number of positions from which power can be delivered and that imply the utility of these other methods (e.g., Snake Creeps Down (Xia shi), Step up to Seven Stars, Golden Rooster Stands on One leg, many of the Sword and Saber postures, etc.).
As for the issue of pulling, I think some good postures to examine would be the transitions into Needle at Sea Bottom, Apparent Closure (Ru feng si bi), and Repulse Monkey. For instance, can our various theories explain the structural usage that happens between Golden Rooster and Repulse Monkey, where the transition and pulling power begins with the weight only one leg?
Michael, I agree with most of your comments. In thinking about your ideas of structure, would your test work equally well if you drastically shortened your stance and your rear toes were almost in line with your forward heel (still maintaining the shoulder width as the lateral distance)?
Jeff, thanks for the explanations. I agree with many of your statements. I absolutely accept the concept of “impressionistic descriptions of experience” and that these often express different physical realities. I, however, still remain attached to modern ideas of physics and so hesitate to try to implement descriptions that appear to violate these, unless I am very sure what kind of feeling someone is trying to get at. I am unsure if you and I are actually discussing either different physical realities or even different subjective realities, but I suspect that there is a true difference in technique.
Earlier in my practice of Taijiquan, I think I used to shift weight by physically trying to sink into the empty leg and then lifting my back leg with minimal exertion of the leg muscles. I am not sure this is the same as what you have explained, but it seems comparable. Two things I never understood about this technique is how one can do this without having to change the height of the body independently of what the opponent is doing and how one can do this with sharpness and speed, since it seems to rely on the limited speed of gravity. For instance, how would one use a full leg to leap into the air without “pushing against the ground”? If, on the other hand, a “pushing feeling” is acceptable here, why would it not be acceptable in Brush Knee and Twist Step?
Psalchemist, you asked about what happens to the Bubbling Spring in postures where this is not in contact with the ground. My understanding is that you root through whatever is available.
If it is consistent with your purpose, keep both your feet flat on the ground. If it is not, root with whatever is available for as long as you can. Diagonal Flying (Xie fei shi) will probably not be done at speed the same way you would do it in form; nevertheless, you want to reach for the same feelings of rootedness to the extent possible. Remember that some or all styles of Taijiquan even include leaping techniques where nothing is contact with the ground. Even here, I believe you reach for the same connections with the ground while you are in the air.
You specifically asked about rooting through the left heel during Diagonal Flying. I would think you certainly do this; however, if you leave things there I think you can run into trouble. If your balanced were threatened by the opponent during the technique, you would probably not insist on keeping the sole of your foot in the air and would end up aborting your technique and rooting with the whole foot flat on the ground. Since this possibility is inherent throughout the technique, I think that you never really entirely abandon the connection through the Bubbling Spring and simply lift your foot enough to allow for easier rotation. Another way of looking at this is that you do not maximize the flex in your ankle to make the heel’s contact with the ground prominent.