Tai-Chi essential 3 of 10: 松腰 (Waist with chi)
松腰 腰为一身之主宰，能松腰然后两足有力，下盘稳固；虚实变化皆由腰 转动，故曰：“命意源头在腰际”，由不得力必于腰腿求之也。
My rendition of essential 3: “One must loosen one’s waist”. The waist is the control part of the whole body. If one’s waist can be loosened, one’s feet can generate power and the body being stable. The tai-chi form should be delivered through waist movements. And the combined effect of the feet and the waist carries the whole form.
Paul’s additional comment: Loosening one's waist is achieved by another important action called 開跨 (opening one’s pelvic joints), a good practice of which can also cure one’s back pain. Only after the pelvic joints can initially be loosened (say through zhan Zhuang) can they be further loosened and with surrounding muscle tendons strengthened through form practice.
In relation to essential 1 and 2 above, this essential act, if done properly can allowed chi generated by the feet to pass through one’s waist without obstruction, and in the process combining with chi generated through pelvic action, directed by the dantian, up the spinal cord and the back and transmitted to the hands, down to each finger (as well as to the whole body) in the process.