Tai-Chi essential 8 of 10: 内外相合 (Mind and muscles as one)
内外相合 太极拳所练在神，故云：“神为主帅，身为驱使”。精神能提得 起，自然举动轻灵。架子不外虚实开合; 所谓开者，不但手足开，心意亦 与之俱开，所谓合者，不但手足合，心意亦与之俱合，能内外合为一气， 则浑然无间矣。
My rendition of essential 8: “During practice, the whole body’s muscles must be felt as one, and the mind must be in synch with this muscle oneness.”
Paul’s additional comment: to create a body with the feeling and applicability of muscles-as-one is the foundation of any style of Chinese internal martial art form. The first step is to create this result during zhan Zhuang (standing meditation). And only when it can be achieved during stationery form can a practitioner be able to transfer this feeling into movements, through diligent practice. And in order to achieve any result, a focused mind is essential during all stages of practice for this essential.