Audi,
I wrote to you in the earlier posting, "With one exception your questions and ideas dance around the answer." And I said that I'd deal with that exception in another post.
The basic response is that weighted pivots per se don't cause or cure double-weightedness. This isn't double-weightedness; it's simply weightedness.
You wrote, "...The logic of this would seem to be that one should always unweight a moving foot to give it nimbleness; however, this seems to run counter to the apparent tolerance or perhaps even fondness of some of Yang Cheng Fu’s successors for weighted foot pivots. If memory serves me right, Wu Yu Xiang Style also has many weighted pivots in changing angles in cat stances (xu shi bu). Can anyone reconcile these practices with these statements about nimbleness?"
Getting caught flat-footed is one thing, and how you pivot is important, but I don't see how a weighted pivot can cause double-weighting.
I don't find pivoting on a weighted foot in any way limiting nimbleness. In fact, in practice, I find weighted pivots increase my mobility. In addition, they are more direct. In some situations if I want to pivot, to turn and go in another direction, I'm not required to shift my weight from the foot, then pivot, then shift the weight back onto the foot again. Why do three things where one is sufficient?
Since you can change the amount of weight on the foot during the pivot, I don't see what the problem is. If you're trying to just turn the weighted foot, that can be hard. Turn your whole leg.
During the long form much of your time is spent with the majority of your weight on one foot, do you really not want to know how to pivot where the majority of your weight is in contact with the ground? I think that there is an increase of agility to be found here. Once familiar with doing weighted pivots correctly, you may see what I'm talking about.
That being said, in other situations unweighted pivots are appropriate. I do both weighted and unweighted pivots. I find that how they are used depends largely upon timing.
Note: so far as I know Yang Tai Chi doesn't insist on either weighted or unweighted pivots. Whether you use weighted pivots or not is your choice. This would be a good question for Yang Jun.
David Salvia
