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

心為令,氣為旂,腰為纛.

武禹襄

"The intention is like an order, the Qi is like the message, the waist is like the banner."

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.