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 with opponents and hands first make contact, concentrate on listening to your opponent's energy. Your task is to follow your opponent and not to do as you please. You must know your opponent and not allow him to know you. Once I understand the opponent, I can easily draw him in regardless of direction so that his energy lands on nothing. In this way, my opponent goes against the flow, whereas I go with it. Here the critical principle is to relax the shoulders. The control mechanism is in the waist and the root in the feet, but they take orders from the mind. With each movement everything moves; in stillness all is still. Above and below are united as with one chi This is what is meant by "stand like a balance and turn as easily as wheel." Controlling the eight directions, you will be invincible. Striking the opponent when his attack is imminent but has not yet issued forth is called "striking the contained chi."Awaiting the opponent's touch in stillness and striking after his attack is already under way is called "striking the incoming energy." Striking the opponent after he has landed on emptiness and seeks to change his energy is called "striking the retreating energy." If you study these principles and pay attention to practice, you will naturally gain complete mastery and gradually attain spiritual illumination.
Lost T'ai-chi Classics from the Late Ch'ing Dynasty, 1996 by State University of New York Press by Douglas Wile.