Tai-Chi essential 4 of 10: 分虚实(The Yin-Yang body with chi)
分虚实 太极拳术以分虚实为第一义，如全身皆坐在右腿，则右腿为实，左 腿为虚；全身皆坐在左腿，则左腿为实，右腿为虚。虚实能分，而后转动 轻灵，毫不费力；如不能分，则迈步重滞，自立不稳，而易为人所牵动。
My rendition of essential 4: “During form practice, one must take note of which leg should be the foundation leg and which leg should be the supportive leg at all time”. The concept of foundation leg and supportive leg is the first rule of tai-chi practice. When the full body weight is on the right leg, the right leg is called foundation leg, and the left leg is called supportive leg (to keep one’s stability). And it is the same for the other side. If one can thus manage one’s weight over one’s legs, one can move forward and backward easily. If can’t, one will lack mobility, and can easily be leveraged by one’s opponent during combat.
Paul’s additional comment: please refer to my earlier post on the subject, both for health and for combat (click here). For physical health benefits, this kind of weight distribution can help train the left and right side separately, resulting in a more balanced and aligned body structure. For combat, it is a fundamental skill (e.g. when throwing a right punch, one’s full weight must be on left leg, with the right leg as stabilizer).