Thanks for the comments.
Michael, the practice you describe is exactly what I try to follow for normal form practice, but I confess to some confusion as to how some of the transitions might or might not be adapted to rapid application.
David, as I understand it, the Yangs describe the transition from the Arising/Beginning Posture (Qi Shi) to Ward Off Left by saying to shift the weight slightly to the left and then to use the waist to pivot on the right heel. As far as I recall, they do not describe any weight shift during the pivot itself, but I guess one could imply a subtle one because of the waist action. One does not sink, bend the right knee, or shift 100% of the weight onto the right leg until after the pivot is complete. Would this fit your description of a dynamic pivot, or is there a weight shift during the pivot itself that is important?
Also, David, do you perform the Turn the Body and Kick with Right Heel (performed near the end of the Second Paragraph, right before the abbreviated Deflect Downward, Parry, and Punch) with a heel pivot? How about the right foot in Turn the Body and Sweep the Lotus (before Bend the Bow and Shoot the Tiger)?
How about the transition between Brush Left Knee and Brush Right Knee, do you pivot on the heel with no rearward weight shift? If so, doesn't lifting the front of the foot cause unusual strain on the left ankle?
Gene, my problem with the spins is more with executing them with Taiji feeling than with the applications per se. However, the only spins I recall from my long-gone Karate days were low sweeps performed on what I would now call the bubbling spring/well, and never on the heel. This is like Turn the Body and Kick with Right Heel performed near the end of the Second Paragraph.
For many years now, I have been trying to concentrate throughout the form on rooting through the bubbling springs and relishing the continuous feeling of stability when I do so. But twice in the form, I have to lift this spot off the ground without being certain what I am supposed to be replacing it with. I do not really find rooting through my heel to be an option.
While I am spinning in these two postures I feel like a balanced stick at best, with little ward off energy, with little distinction of full and empty (other than in my foot), uncharacteristicly committed, and with little ability to channel energy from an opponent. I frankly feel stronger and more stable in mid leap during the sword and saber forms, where an attack from my opponent would trouble me less.
If I ignore Taiji constraints, I can speculate on many ways to "improve" my stability during the two heel spins: for instance, bending my supporting knee, crouching, hopping, separating the spins into discrete parts, using speed, spotting with my eyes, jumping into position, turning my body in discrete sections, etc. One alternative would be to use a quick pivot aided by the ball of the non-supporting foot (as seems frequent in Wu/Hao Style and as David has described as one option for Turn the Body and Kick with Left Heel).
On the very few occasions I have pushed hands with leg involvement, I don't recall ever feeling inclined to lift the bubbling spring of my supporting leg off the ground, unless my non-supporting leg was touching and substantially involved with my opponent. Alternatively, I would move my body in sections, first aligning my torso and the non-supporting leg, and only then allowing my supporting leg to pivot into the new position.
Just to be clear, I am not advocating any of the work-arounds I describe above, but just trying to be honest about what I feel.