Hi David, Michael, and Jerry:
David, I think I now understand enough about my own practice and your descriptions to comment about how you characterize waist movement. In short, I think there may indeed be a slight difference in what you do from what I do, and probably from what Jerry does, from what I recall.
I understand the waist to refer to the soft tissues around the lumbar spine. If I am sitting on a stool, I can still rotate my Taiji waist right and left without moving my hips. If I am standing up and someone immobilizes my shoulders, I can still rotate my waist without moving my shoulders. In both cases, I am centering movement in the same part of my body. If both my hips and shoulders are immobilized, I can still generate slight waist rotation, but this is almost invisible and probably only apparent in looking at my abdominal muscles, where one side contracts inward while the other side protrudes.
Subjectively, I can see how this motion could be described as simultaneously moving the shoulders one way and the hips another, but I think in reality, it is a question of using the waist to move either the hips or the shoulders or moving both together in the same direction.
My Taiji waist cannot simultaneously move my hips one way and my shoulders the other way, although some of this may only be semantic. Often where you describe yourself as doing this (such as after White Crane Spreads/Dries (its) Wings), I think I (and probably Jerry too) am not moving my hips or lower body much at all, but am using my waist to carry along my upper body to the right. Even though the lower body does not move, there is still a relationship of dependency since the torque of the waist affects the feeling in the hip joints (“kua’s”).
Michael and Jerry, when I first began trying to follow the Yangs’ teachings I struggled greatly with what they meant by “using the waist.” One of the accommodations I have reached is that I never mentally associate the motion of the leg stepping out with waist movement, even if in some postures there is some simultaneous movement. I think of this as a “blind” and “empty” step.
I think Yang Zhenduo described this kind of step in a seminar be referring to "going left when one wants to step to the right, etc." I think he also referred to something like this as a requirement for making the steps free and nimble.
A clear instance of this principle is in Diagonal Flying, where the right leg steps out to the right rear (southwest) while the waist is still facing east. Jerry, I think this what you described in your post above and what the Yangs have described in the seminars as “opening the hips.” In other words, one opens the hips to perform the step, but reserves the waist movement for the upper body. This may also be an instance of what David describes as moving the shoulders one way and the hips another, although I would describe this movement as leaving the shoulders and waist in place while the hips open to the right. The first step with the right foot in Deflect Downward Parry and Punch is another instance of this idea, where the step lands with the toes pointing to the southeast while the torso is still facing northeast.
I have found this principle of what I am here calling “blind empty stepping” to be important, because I reason that if I use some waist torque to first power my step and then to power my upper body movement, I have half the waist torque available to power to my upper body. In addition, the step itself is one of the few components of the movement where central equilibrium is the paramount quality necessary, and no power needs to be generated in the stepping leg. For me, this is another instance of distinguishing full and empty, because using the waist to power my step, rather than something else, would be a violation of this.
In contrast, it seems to me that the Yangs power even the seemingly most trivial heel pivots with the waist. Yang Zhenduo always seemed to describe these situations by saying that the waist carries the foot along to turn inward or outward.
In applying all of this to the ward off to the left near the end of Single Whip, it seems to me that one would want to maintain at least two principles, which you all have already touched on. One should not ward off until one has root in both feet, and one should reserve the waist for feeding power to the warding arm and not use it to power the step to the left.
If one observes these two principles, one is still left with the issue of continuity. I think this touches on another dichotomy that Yang Jun discussed a little at my last saber seminar, which is the clarity with which one differentiates weight shifts versus the continuity one gives to movement. I got the distinct idea from him that these were variable concepts, but that clarity was more important in the bare-hand form and that continuity was more important in the saber or sword form. In short, I think that the waist turn and step should probably be two distinct movements, but that some blending is okay if there function is kept mentally distinct.