The below quote is taken from a popular quanzhen document called “Pointing to the Way of Elixir” which is a meditation document that explains many fine aspects of the energy anatomy of the body and how to meditate, The theory here is both simple and complex, but it is essentially concerned with the routs along which Qi energy rises and falls in the body. There are notes at the end in order to help explain some of the more complex points.
夫背后尾闾、夹脊、玉枕，谓之三关，属督脉，为阳；
the Wei lu, Ni Ji, and Yu Zhen gates run up the back and are called the Three Gates. They correspond to the “Du Meridian” and are considered as Yang.
前面上丹田、中丹田、下丹田，谓之三田，属任脉，为阴。
The three Elixir Fields in the front of the body are the upper elixir field, middle elixir field, and lower elixir field. They are part of the Ren Meridian and are considered Yin.
此阴阳升降之路，自背后督脉上来，即属子，
This is the road along which yin and yang rise and fall.
When the Du Meridian rises up the back, it is called “Zi”
自前面任脉下去，即属午，
when the Ren Meridian moves down the front, it is called “Wu.”
子午抽添，所谓周天火候是也。
Zi and Wu can be added and withdrawn from, and are the rout by which “Huo Hou” can be sent around the heavenly orbit.
Note:
Zi and Wu are two ancient Chinese concepts of time which infer the rise of the moon to its peak (Zi) and the rise of the sun to its peak (wu). Zi represents the water element and the kidneys, as well as the Du meridian, running from the base of the spine, past the kidneys, up the back, and neck, and to the head. Wu represents the fire element, the heart, and the line of energy running down the front of the body from the upper Dan Tian to the “Sea of energy” in the lower abdomen.
The reason why Zi can be considered as yang energy and Wu can be considered as yin energy is because of the centre lines of the fire and water element. The centre line of the three lines making up the water trigram is solid, indicating living yang energy and human essence. The broken line at the centre of the three lines of the fire trigram represents yin energy and to oxygen that moves in our blood, as well as our emotions. So even though fire is predominantly a yang energy, its inside is yin, and the time of “Wu” in the early afternoon indicates the time when the sun is about to pass its Zenith at the time of “solar noon” and is about to overextend itself and begin its descent, this means that yang is getting ready to turn into yin. The same is true of the moon and the time of Zi at midnight. This theory is substantially more complex, but for the time being, we can simply work on the assumption that the Du meridian is a yang energy conduit working on a yin part of the body and that the Ren Meridian is a yin energy conduit working on a predominantly yang part of the body.
Huo Hou: Huo Hou, or “Controlling the fire” is the act by which yin and yang are stabilized in the intention in order to subtly control the Qi as it moves in the body. this is mainly concerned with the way in which we set our intentions and how we can gradually stop the mind and not readjust it during meditation. The best situation is that the mind remains stopped most of the time and only has to be infrequently adjusted, but this level of practice can take a long time to achieve. Huo Hou is how we learn to keep the fire of the mind alive without allowing it to become too intense or go out. This is a very subtle concept in Daoism.