Excerpt from Important Explanations for the Accomplishment of the Thirteen Postures (十三勢行功要解) by Wǔ Yǔxiāng (武禹襄)

氣以直養而無害,勁以曲蓄而有餘.

武禹襄

"The use of Qi is reasoable cultivation, in this way it is harmless; the use of Jin is to be coiled and stored up, in this way one has it in abundance."

By Wǔ Yǔxiāng (武禹襄), based on the translation by Yang Jwing Ming (楊俊敏) presented in the book Tai Chi Secrets of the Wu & Li Styles (太極拳武李氏先哲秘要) and various translations on the internet.