Audi and David: At the risk of stating the obvious, there are some very clear weighted pivots in the form as done by YZD (e.g., transition into double wind blows ears, turn into the sweeping lotus kick near the end of the form). When I do these pivots, I think not of the support leg, but of the support foot. In other words, part of the weighted foot (ball or heel) must become yin to allow a smooth turn. If the balance of yin and yang in the foot is discombobulated (how's that for a word?) so is the player's body. Result: either a wobble (discombobble?), or lost balance. For me, that translates into the overarching no-no of "double-weightedness." On the subject of the pivot after fan through back (while there are many applications and views of everything, as you know) try looking at the non-weighted pivot as the setup for a chin na technique, instead of a turn to meet an attack from the rear. That is, get a partner, and have him or her throw a right straight punch or push at your head (slowly) at relatively long range, with right foot forward. Deflect from the outside with fan arm positioning, then slide your right hand back and grab the opponent's right wrist while you take weight off your left foot and shift weight to your rear to cause an uproot (lu energy). Keep your elbows and shoulders sunk. Be careful - since he's already coming forward this can work better than you would think (I know this from personal experience). Also, you have to be very careful to avoid pulling the opponent directly into you - make sure you are pulling on an oblique line to your right rear, or this technique will fail. Keep your left hand on the back of the opponent's right arm as a guide and to prevent getting belted by an elbow. (Note: In application practice or freestyle practice, depending on your partner's size and distance, you may have to take a step back with the rear leg to make the uproot work most effectively.) Once you have the opponent uprooted, make your waist turn, and twist the opponent's arm over and down with your right while pressing your left palm downward, with force, to the back of the opponent's right shoulder. Make sure that when you end up, the opponent's wrist is higher than his shoulder, which will give you a nice, firm lock with your right (now unweighted) foot in position to cause some serious damage. You will also be in virtually the exact position you would be after the turn in the form, except your left hand will be lower, and your right palm may be face up rather than face down, to get the proper torque on the opponent's arm. (By the way, I was once at a Yang Jwing Ming seminar where he defined the purpose of chin na as "putting an opponent in position to kill." Yikes.) Fool around with this a few times and you'll eventually hit the lock and feel what I mean - you need to shift back and make an unweighted pivot to cause the uproot and make the technique work, then the rest flows effortlessly. (It's relatively easy to hit the lock when your partner is distracted by trying to regain his balance.) So: I think the difference between using the weighted and non-weighted pivot on the transition depends on the application you're noodling through. And to me, answering these kinds of questions always starts with thinking of combat applications, and reasoning them back into the form. This is why, even for those who are only interested in health benefits, I think that knowledge of applications, and practice of applications, is vital.
Other than that, I have no opinion. And by the way, Audi, please e-mail me as I would like to see whether we can get together once in awhile to practice. We talked this summer in Montreal and you don't live far from me.