Excerpt from <span class="chinese-post-title">十三勢行功要解</span> the Important Explanations for the Accomplishment of the Thirteen Postures by <span class="chinese-post-title">武禹襄</span> Wǔ Yǔxiāng

精神能提得起，則無遲重之虞。所謂頂頭懸也。
意氣須換得靈，乃有圓活之妙。所謂變轉虛實也。

"If one is able to raise the spirit of vitality, then there is no worry of being slow or clumsy. This is also called the top of the head is suspended. Yi (Intention) and Qi must exchange quickly; so the wonderful liveliness of the circle is present. This is also called the substantial and the insubstantial can transform in each other."

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.