Loose the waist (鬆腰) from The Ten Important Points of Tai Chi Chuan (太極拳十要)

鬆腰

腰為一身之主宰，能鬆腰然后兩足有力，下盤穩固﹔虛實變化皆由腰轉動，故曰︰«命意源頭在腰隙»，«有不得力必于腰腿求之也»。

楊澄甫

"The waist is the master of the entire body. If one is able to loosen the waist then the his two feet have power and the lower disk is steady and firm. The variations of insubstantial and substantial all originate from the waist's turning and movement. Therefore, it is said: «the origin of the life and intention (Yi) is in the waist.», «If one is in a place where it is not possible to use force (Li), then a solution must come from the the waist and the legs»."

By Yáng Chéngfǔ (楊澄甫), based on the translation by by Yang Jwing Ming (楊俊敏) presented in the book Tai Chi Secrets of the Yang Style (太極拳楊氏先哲秘要) and various translations on the internet