Q&A on the Contemplative Practice of the Four Establishments of Mindfulness, 27

Question: How should one practice contemplating the breath? 

Answer: When contemplating the breath, you should put all your energy on the breath, and not concern yourself with what will happen to the body, or whether a yang spirit (yangshen) will emerge in the future, and so on. The Daoist yang spirit has nothing to do with us; there is too much that does not need to be understood. It is enough for us to understand the influence of qi channels (qimai) on the physical body—just knowing this theory is enough. Do not concern yourself with the yang spirit; we are not cultivating the yang spirit, but cultivating the mind, in order to attain realization and eradicate the view of self (satkāya-dṛṣṭi). As for what may happen to the body, a slight understanding is enough; knowledge of body-cultivation that is too deep or too extensive has nothing to do with our cultivation of the mind, so do not spend energy on unnecessary things.  

We study the four foundations of mindfulness (satipaṭṭhāna), and at present we focus mainly on contemplating the breath. Apart from the breath, pay as little attention as possible to other aspects of the physical body. Keep your attention only on the breath. When contemplating the breath reaches a certain degree and your meditative concentration (samādhi) strengthens, you will discover the objective truths of the physical body's emptiness, falsity, impermanence, and non-self. If you pay too much attention to the physical body, your attention will be scattered, which will affect your contemplation of the breath. 

When contemplating, try to be objective and observe calmly, without adding subjective consciousness. Whatever the state of the physical body is, just knowing it is enough; do not add excessive notions such as thoughts and guidance. In the beginning you may regulate the breath, but afterwards you should no longer regulate it—just follow the breath. In this way your mind will naturally leap outside the five-aggregate body (pañca-skandha), and then turn back to observe the five-aggregate body and the breath, standing at the highest vantage point and looking back to observe the state of the physical body. On the one hand, this makes it easy to enter meditative concentration; on the other hand, it makes it easy for wisdom to arise, and you will discover many truths. Truths you did not know before, you will now know. Gradually you will come to feel that the physical body is so impermanent, so boring, so illusory, and gradually the wisdom of non-self will arise.