Top and Bottom Mutually Follow Each Other (上下相隨) from The Ten Important Points of Tai Chi Chuan (太極拳十要)

上下相隨

上下相隨者，即太極拳論中所云︰«其根在腳，發于腿，主宰于腰，形于手指，由腳而腿而腰，總須完整一氣»也。手動，腰動，足動，眼神亦隨之動。如是方可謂之上下相隨。有一不動，即散亂也。

楊澄甫

"The meaning of “top and bottom mutually follow each other” is in what the Taijiquan Treatise says: « The root of the movement is in the feet, it develops through the legs, it is ruled by waist, and manifested by the fingers. From the feet to the legs, and to the waist, it must be integrated with one completely unified Qi.» The hands move, the waist moves, the feet move, the eyes direction move accordingly. In this situation it can be said the top and the bottom mutually follow each other. If there is a place that does not move, then it can be called disordered."

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