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

往復須有摺疊,進退須有轉換.

武禹襄

"In going back and forth there must be folding and mutual entwining. In advancing and retreating there must be variation and transformation."

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.