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(吳家太極拳, Wu family T'ai Chi Ch'uan) also known as The Gold Book: by Wu Kung-tsao, the second son of Wu Chien-ch'uan,
I quote Wu Chien Chuan: "The idea of loosening the waist is to make the waist light and relaxed. In Tai Chi Ch'uan all the turnings and shiftings of the body weight are controlled by the waist. In the Song of the Thirteen Postures it is written: " The source of command for all movements lies in the waist". If one's waist is at ease and relaxed, not only can one's intrinsic energy easily sink into the "tan-tien" and flow freely and actively, but the lower part of the body can be strengthened and one avoids having the mistake of having the upper part heavy and the lower part light. To droop the buttocks is to allow the buttocks to sink vertically so that they do not protrude. Particularly when one squats one should pay special attention not to violate these rules. The failure of drooping the buttocks is a hindrance to the loosening of one's waist".