When people start to learn meditation, it is important to recognize the individual phases of practice and how to get the best result from them.
Huo zi shi is a concept used especially often by Huang yuanji which refers to the time when the latent energy of the water trigram first springs to life.
“Zi Shi” in traditional Chinese time means the two hour period between 11 PM and 1 AM and is symbolized by the water element and the rise of the moon. Usually in meditation practice, it is considered one of the best time to practice, because the energy of the moon can help bring out the energy of the body more abundantly. I also suspect it is related to the circadian rhythm and the body being better at going into a deep meditation state at that time, since usually people will rest around this time of night.
Daoists used the term “zi shi” to describe the time when the inner yang line appears inside of the kun trigram, changing it to Kan, or water. “Huo zi shi” means the time when “Zi” comes to life and it is a specific idea in meditation, especially in the middle school.
Huo Zi appears during the turbid, chaotic silence of deep meditation. It is when yin changes and yang emerges. It is the chaotic energy of the pre heaven and is the time when the pre heaven “three treasures” of yuan jing, yuan qi, and yuan shen are all mixing together. During this time, even though your mind and body are present, it seems like you have no body, and no mind. Huang Yuanji says it is like not knowing one’s home village, in other words, it is when the mind goes into the void state.
Because of entering into the void, the body begins to follow its own natural cycle and the conscious mind or “Shi shen” (the mind that looks) has been replaced by the pure mind, or “Yuan Shen” (the original mind). This theory can be very complicated, but to put it in simple terms, the “Shi shen” is polluted by emotions (Qing) and when you are in deep mediation, the emotions disappear and are replaced with clarity. Therefore, it is at this time that the mind enters into a state of pure yang energy and the body enters into pure yin energy. Li Daochun said that this is the time when the inner line from the water trigram moves to the inner line of the Li trigram, clearly changing water and fire to earth and heaven, thus propping up and making clear the separation between the pre heaven, unmixed Yin and Yang, and thus allowing the spirit to emerge as pure yang energy.
This kind of practice has many stages and it isn’t a one time thing, nor does it always manifest the same way. The easiest way to understand it is as a concept about how to swap yin for yang so that the broken yin line in your consciousness is repaired by the solid yang line of your body. This is the way that “Xing” and “ming” merge together, at least in the theory of the middle school of Nei Dan.