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When the heart is not quiet, one may not concentrate. Lifting the hands, moving forward, backward, left, and right, will focused direction. Therefore, the heart must be quiet. From the moment one initiates motion, its not determined by the self. You must quiet the heart and understand with your body. Your movements follow those of your opponent. Follow bending with extending with extending. Do not let go nor resist. Bending and extending are not determined by you. When the opponent is strong I am also strong. When the opponent is weak I am still strong. My intent always arrives first. You must pay attention at all times. Wherever there is contact, there is a focus of concentration. In the midst of not letting go nor resisting you must gather information and proceed from there. After a long period of practice, you will be able to use this information physically. This is completely dependent upon the use of intent and not on force (jing). Eventually, the opponent is controlled by me, I am not controlled by others.

The Body is Agile
If the body is sluggish, one cannot advance and retreat as desired. Therefore, the body should be agile. When moving the hands, one most not be dull. If I feel the opponents power has touched my skin, my intent has already penetrated his bones. The hands support and all is unified in a single qi. If the left is heavy it becomes empty and my right hand has already struck. If the right is heavy the it becomes empty and my left has already struck. The is like a wheel. The entire body most coordinate its individual movements. If there is any part that does not move in concert with all others, the body will be in chaos and powerless. The root of the problem is found in waist and legs. First, the heart follows the body. Follow the opponent and not the self. Later, the body follows the heart while still following the opponent. If one move without following the opponent, movement will be sluggish. If movement follows the opponent, it will be alive. If one follows the opponent, one’s hands will be sensitive and the opponent’s power may be judged exactly. The distance of the opponent’s attack will not be miscalculated even by a hairs breath. Moving forward and backward, advancing and retreating will be appropriate. The longer you practice, the more refined you technique will become.

The Qi is Stored
If the qi is dispersed and not stored within, the body will easily lapse into chaos. The qi should be held in the spine. The breath should be smooth and fill the entire body. Inhalation is closing and storing, and exhalation is opening and releasing. During inhalation one naturally rises and holds the opponent up. During exhalation one naturally sinks and knocks the opponent away. This involves the intent leading the qi and not the strength leading the qi.

The Force (Jing) is Complete
The force of the entire body is trained into a unified whole. Substantial and insubstantial are clearly differentiated. When issuing force, there must be a root. The force rises from the heel, is controlled by the waist, and manifests in the fingers. It issues from the spine. One must also raise all of one’s spirit. Just as the opponent is about to issue force but has not, my force has already intercepted the opponent’s. I must not issue my force earlier or later. Even if you feel as if you skin is on fire or you are struck by a flood, you most not become the least bit perturbed. Seek the straight in the curved; first store the release; only then can you achieve consistent results. This is called borrowing the opponent’s force to use against him, or using four ounces to deflect a thousand pounds.

The Spirit is Concentrated
After allowing the first four requirements, it all comes down to concentrating the spirit. When the spirit is concentrated, then the one qi is stimulated and forged. The essence and qi are returned to the spirit and the qi is active and expansive. The essence and spirit are concentrated. Opening and closing regulated. Insubstantial and substantial are clearly defined. When the left is empty the right is full. When the right is full the left is empty. Insubstantial (empty) does not mean completely without power. Substantial (full) does not mean completely tight. The value of the spirit is concentration. The critical locations are the center of the chest and the waist. Its movements and use is not external. Borrow force from others. The qi issues from the spine. The qi sinks downward; it is pulled in from the shoulders into the spine and concentrated in the waist. When the qi moves downward from above, it is called closing. From the waist the qi moves up the spine and enters the arms. It is issued in the fingers. When the qi moves upward from below it is called opening. Closing is withdrawing. Opening is releasing. To understand opening and closing is to know yin and yang. At this level, power and skill improve daily. Slow, you will com to the state where you can act at will. 

Reference:
A Study of Taijiquan by Sun Lutang, Translated by Tim CartmellISBN 1556434626

p. 216-18