Having the above four, then you can return to concentrated spirit: if the spirit is concentrated, then it is (continuous and) uninterrupted, and the practice of chi (breath) returns to the shen (spirit). The manifestation of chi moves with agility. (When) the spirit is concentrated, opening and closing occur appropriately, and the differentiation of substantial and inubsubstantial is clear. If the left is insubstantial, the right is substantial, and vice-versa. Insubstantial does not mean completely without strength. The manifestation of the chi must be agile. Substantial does not mean completely limited. The spirit must be completely concentrated. It is important to be completely in the mind (heart) and waist, and not outside.
Not being outside or separated, force is borrowed from the opponent, and the chi is relased from the spine. How is the chi released from the spine? It sinks downward from the two shoulders, gathers to the spine, and pours to the waist. This is chi’i from the up to down is called “closed”. From the waist the chi mobilizes to the spine, spreads to the two arms and flows to the fingers. This is chi from down to up and is called “opened”. Closed is gathering, and opened is discharging. When you opening and closing, then you know yin and yang. Reaching this level your skill will progress with the days and can do as you wish.
Red.: from Li Yi Yu’s Five Character Secret (Calm, Agility, Breath – to gather the chi, The internal force – the complete chin, Spirit – Shen concentrated).
Reference: T’Ai Chi Ch’Uan Ta Wen, Questions and Answers on T’Ai Chi Boxing Chen Wei-Ming ( Benjamin Pang Jeng Lo & Robert W. Smith ) North Atlantic Books 1985