Excerpted from the book Tao and Longevity by Nan Huaijin
Does the spirit actually leave the body during the transformation of chi into shen?
…There are many [Taoist] descriptions of being pregnant for ten months, suckling the baby for three years, and facing the wall for nine years that have led some people to believe that successful meditation must involve astral projection. The supposition is that the spirit or divine self has a fetal body of its own which ultimately shoots out of the top of the head and ascends into heaven itself. To believe that this is the way of transforming chi into shen is a serious mistake.
According to the Tan Tao school, yang shen (or positive spirit) and yin shen (or negative spirit) may both account for the projection of the spirit from out of the body.
Yang shen can leave this body of flesh and blood, and yet it is a physical body with material form and a visible appearance. It can behave exactly as a normal human body behaves and yet it is a spiritual body with a physical form and an independent existence. It is believed to be a projection of the real spirit.
A person who says that he has an invisible and immaterial body, with or without form, that leaves and enters his body at will is speaking of the body of yin shen. Yin shen is similar to those bodies appearing in dreams, but it is much clearer and more distinct than ordinary “dream” bodies. [Some] Taoists maintain that the projection of yang shen is the ultimate achievement of meditation and the cultivation of Tao. But if one imagines that the projection of yin shen is the ultimate goal of Tao, then he is an ordinary person or in the yin state of a spirit or a ghost.
Self-Deceptions and Semblance Dharmas
Nan Huaijin (Huai-Chin Nan)
…Some may have an experience of this sort long before reaching the stage of transforming the chi into shen. For example, when the connection between the spirit and the body has deteriorated and grown weak, the projection of yin shen may occur spontaneously. This can happen to quite ordinary people who are not meditating, for it happens when one is ill, nearing death, or bordering on schizophrenia.
Sometimes people with nervous temperaments who engage in the practice of meditation may experience the projection of yin shen. This phenomenon is often influenced by the subconscious mind, when it is attuned to the chi ascending to the head. A person who is well versed in psychology, and who introspectively reflects upon his subconscious mind need not deceive himself about this…
Is the cultivation of shen a valid process? Theoretical research, as well as firsthand experience, convinces me that the cultivation of shen is valid. However, to participate in this process, one should be completely his own master.