Wu Ju-ch'ing was Wu Yuxiang's older brother. This text was uncovered recently, and is reproduced below. This martial art is called "t'ai-chi" because it is based on yin and yang, full and empty. After one is clear about yin and yang, one can begin to understand advance and retreat. Although advance means to advance, it must contain an awareness of retreat; to retreat is still to retreat, but it conceals an awareness of the opportunity to advance. Both of these hinge on the ability to open the energy at the crown of the head. This together with raising the back and relaxing the chest allows the spirit to rise. By sinking the chi to the tan-tien and protecting the crotch and buttocks, one can move freely and with agility. Bend the elbows, for from a bent position you can extend them and control the situation to your advantage. Bend the knees, for from the bent position they can extend, and thus when issuing energy you will have power. When it comes to sparring