Use Intention, not Force (用意不用力) from The Ten Important Points of Tai Chi Chuan (太極拳十要)

用意不用力

太極拳論云︰«此全是用意不用力。»練太極拳全身鬆開，不能有分毫之拙勁，以留滯于筋骨血脈之間以自縛束，然后能輕靈變化，圓轉自如。或疑不用力何以能長力?蓋人身之有經絡，如地之有溝洫，溝洫不塞而水行，經絡不閉則氣通。如渾身僵勁滿經絡，氣血停滯，轉動不靈，牽一發而全身動矣。

楊澄甫

"The Taijiquan Treatise says: « All of these are using the Yi not the Li » (When) training Taijiquan, the entire body is loosening and opening, cannot have (even a) centimeter or millimeter's (i.e., slightly) clumsy Jin which (may cause) self bondage (i.e., clumsiness) due to the delay and stagnation (of the Qi and blood circulation) within tendon, bones and blood. Then, (you) can be light, agile, varied, and round as wished. (You) may doubt, how can you grow power without muscular force? It is because the Jing and Luo (i.e., primary and secondary Qi channels) in the human body are like streams and ditches. When the streams and ditches are not blocked, the water will flow (smoothly), and when the Jing and Luo are not shut off, the Qi will transport (easily). If there is a stiff Jin (applied on) entire Jing and Luo (system), the Qi and blood will be stopped or stagnant and the turning and movement will not be agile. (When this happens,) once a place is pulled, the entire body moves. "

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