Tai Chi Chuan Ching by Chang San-feng

The Chi (breath) should be excited.
The Shen (spirit) should be internally gathered.

We breath continuously all day and night. Throughout our normal activities we rarely think about it. When students initially learn various breathing methodologies like abdominal and thoracic breathing, they erroneously abandon their naturalness and become forced and mechanical, making loud noises and stiffening their body. Thinking the objective is pumping more air in, they go down the wrong path.

Natural breathing is restricted when we do not softening the body and yield to the internal pressure being created inside our body as air fills the lungs. Greater range of motion (opening the body) is achieved as the body continuously softens and expands, yielding to the internal pressure. Air is not forced into the body like blowing up a balloon, but a study of anatomy will reveal the diaphragm draws the lungs down during inhalation, sinking the Chi down to the Dan Tien. During exhalation the diaphragm comfortably ascend to a relaxed position. It is feeling (sensation) of the internal movement inside the body that is the focus of all internal studies.

If the body is still (sitting and standing), breathing will naturally become long and slow. As activity is increased through physical movement (stepping, kicking, leaping, etc.) a greater demand for oxygen is required by the body causing a shorter and faster respiration rate. The point to breathing is, just like in form practice, discovering and correcting impingements that cause blockages to the body's natural movement is the objective, and not mindlessly forcing a change to simply meet a desired objective.

As the mind focuses on the immediacy of full awareness on the practice, "letting go" of any discursive thoughts as they appear in the mind, the Shen (mind - logic and heart - emotions) unifies. The Yi (Will / Intention) is totally absorbed in the work at hand.

In essence, this is the mind and body blending together becoming one functionally. A combined strength of mind and body implies using intention over force. When Form (quan) follows Function (gong - skill) the energy manifests naturally. How can there be peace of mind, if the mind is lost in the fundamental study of remembering choreography?