You wrote in an earlier post:
“. . . I'd like to hear what you say about the fact, that when doing single whip in the left bow stance manner you've described, the left foot and the left hand is substantial (other than in Brush knee left). I've always learned, that if left foot is full, left hand is empty and vice versa. Here and in other related postures (e. g. Fan through back, High pat on horse)it seems as if this principle doesn't work. I was asked sometimes for this, but I cannot really answer to it”
I’ve heard that too, about the supposed necessity of “left foot full, left hand empty,” but I can’t seem to figure out a practical or logical basis for adhering to that prescription. Where did that come from, anyway? I think Zheng Manqing may have written something to that effect, but I can’t say that I’ve ever seen any support for it in the taiji classics. In fact, while it may be a helpful provisional or intermediate guideline for “clearly distinguishing full and empty,” however, as one’s practice develops, I think it could actually hinder advancement. Holding to such an inflexible interpretation would not, in my mind, accord with classical yin/yang theory (which Audi brought up in a recent thread).
There is an early taiji saying, “within empty there is full; within full there is empty.” The “Taijiquan Jing” also states, “Each point has its point of empty/full. Everywhere there is always this one empty/full.” When first learning the form, the challenge is to distinguish between a full arm and an empty arm, but it does not stop there. One must come to understand that full and empty can be distinguished in one arm, wrist, finger, palm, leg, sole of the foot, etc.
In Single Whip, it may be helpful to think in these terms. As you say, the left arm and palm is full with regard to concentration of jin and intent. While this is a left-weighted stance, the left foot is “full” only in the sense of bearing most of the body’s weight. In another sense, it is empty, as it is only carrying the weight, but it is playing a subsidiary role with regard to directing the intent of the strike. The right foot could be said to be empty in the sense of its share of the weight being born, but it is full in the sense that it is “charged” with jin. One can sense a charged conduit from the right heel directly into the end of the left arm’s ulna at the base of the palm. Here, I find a saying from Li Yiyu’s “Five Key Words” particularly useful: “Empty does not mean completely devoid of strength (li), and full does not mean to completely stand firm (zhan sha).”
A good while back there was a discussion on the board about the terms “ao bu” (as in Brush Knee Twist Step), and “shun bu,” which has been translated as “favorable step” in Huang Wen-Shan’s 1973 book, _Fundamentals of Tai Chi Ch’uan_. Huang explains that when the forward hand and forward foot are both on the same side, it is called “favorable hand.” When the forward foot and forward hand are on alternate sides, it’s called “twist step” (ao bu). I discovered this exact explanation in Xu Yusheng’s 1921 book, _Taijiquan Shi Tujie_ (Illustrated Explanations of Taijiquan Postures), which Huang evidently cribbed for this and some other material in his book. The term Huang was translating as “favorable” is “shun” which means “goes along with,” “in the same direction with.” One of the glosses one will find in Chinese dictionaries for “ao,” is “bu shun,” that is, “not” shun. So it is the opposite of “going along with.” Another meaning of shun is “easily, smoothly.” Shunshou means it comes easily to the hand: easy and convenient. In any case, the fact that we have these instances of “shun bu” and “ao bu” in the form should point up the fact that an overly formulaic interpretation of where and how “empty” and “full” are distributed in the forms should be avoided.