Rubén, after reading my earlier post, I realize that I may not have been as clear as I should.
I think we have been discussing two distinct cases: taking a full step back (where the front leg moves past the back leg to become the new back leg) and taking a half step (where we turn a bow stance into an empty/full stance with the same foot forward.
Let me describe the full step first.
As far as I know there is no absolute requirement that determines how the original front foot needs to leave the ground to begin the step. If only the ball of the foot is touching (such as in Ape Presents the Fruit or in Step UP to Seven Stars), you merely lift the foot and step back. If the foot begins flat to the ground (such as in the 2nd to the 5th repetition of falling flowers or the 2nd and 3rd Repulse Monkey), you first lift the toes to unlock the root, then you lift the ball of the foot, and then you push off with the heel. The front foot generally only has to move a little/moderate amount of weight to the other leg.
The original back foot has to receive the weight from the original front foot. Then, after the old front foot touches the ground in the back, when a little weight is then shifted to this new back foot in order to flatten it and create root between the legs, the new front foot must shift a moderate/great amount of weight in order to help power the Fajin (i.e., issue power). If the new front foot must be adjusted as you issue in order to keep the posture comfortable, it seems to be more powerful and more unified to pivot on the heel then on the ball of the foot. Some other teachers, however, seem to have a different understanding.
When we take a half step, rather than a full step, we also want to shift weight as we issue power (i.e., Fajin). For us, empty and full does not refer to all weight or no weight, but rather to more or less weight. If we intend to form a posture with the front heel on the ground, we prefer to get there by having first left the ground using the ball of the foot. Conversely, if we intend to end up with the
ball of the foot on the ground, we prefer to have previously left the ground using the heel. This alternation helps the energy remain continuous and to respect the Yin/Yang alternation. (E.g., if you want to go right, first go left, etc.)
In at least one posture (i.e., Waiting for the Fish), alternating the touching point is too complicated a movement and too unnatural, and so we leave the ground using the ball and retouch with the ball. Even so, we still withdraw some weight from the right foot and then shift it right back at the end of the posture. In almost all cases, we avoid trying to do Fajin without a simultaneous shift of the weight and avoid issuing power with only one foot rooted. (Kicks are another matter, with another explanation.)
By the way, the weight shift in the full/empty stances is not simply a case of a contingency plan in case you happen to miss the technique. The weight shift is an integral and extremely powerful aspect of the technique. If we take Step UP to Seven Stars as an example, with the last half inch of weight shift and the slight changes in the knees and ankles after the ball of the right foot touches, you should be able to punch very hard using whole-body Jin.
Another thing I should say is that I agree with what Bob said about close-in attacks with the sword and saber. That is why I said that you do not want to commit to a close-in technique without being able to back out quickly, such as in the transition into the Little Dipper. Sinking into the front heel would make withdrawing it a fraction slower than touching with the ball of the foot. Remember that the front leg is a principal target and so needs to be very nimble.
That's all the time I have for now, but I hope this helps.
[This message has been edited by Audi (edited 03-09-2009).]