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

極柔軟,然後極堅剛.能呼吸,然後能靈活.

武禹襄

"Extremely soft, then extremely hard. If one is able to breath properly it is possible to be nimble."

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.