Precelestial breath, Postcelestial breath. Those who obtain them always seem to be drunk.

The precelestial Breath is the original and initial Ancestral Breath.1 This Ancestral Breath is in the real center of Heaven and Earth within the human body. [Placed between] the Secret Door and the Gate of Life, hanging in the middle, it is the Heart of Heaven.2 The self-cultivation of the divine Immortals only consists in collecting the precelestial One Breath and using it as the Mother of the Elixir.
The postcelestial Breath is the Breath that circulates internally: one exhalation, one inhalation, once coming, once going. “Exhaling touches onto the root of Heaven, inhaling touches onto the root of Earth. On exhaling, ‘the dragon howls and the clouds rise’; on inhaling, ‘the tiger roars and the wind blows.’”3
When [the postcelestial Breath] is “unceasing and continuous,”4 it returns to the Ancestral Breath. The internal and the external inchoately merge, and coalesce to form the Reverted Elixir (huandan). Then you become aware of a burning fire in the Cinnabar Field that spreads to the four limbs. You look like a fool or like drunk, but “its beauty lies within.”5 This is why it says, “those who obtain them always seem to be drunk.”
This is what the Daode jing (Book of the Way and Its Virtue) means when it says:

The Spirit of the Valley never dies:
it is called the Mysterious-Female. The gate of the Mysterious-Female
is called the root of Heaven and Earth.
Unceasing and continuous,
its operation never wears out.6

And this is what the Book of Changes (Yijing) means when it says about the Kun ䷁ hexagram:
From the Yellow Center it spreads to the veining, as it places itself in the correct position. Its beauty lies within, and extends to the four limbs.7

Notes
1. “Precelestial” (xiantian) and “postcelestial” (houtian) refer to the states before and after the generation of the cosmos. The precelestial Breath (qi) is the One Breath of the Dao. Once the cosmos is generated, it is permeated by the postcelestial Breath, which manifests itself in the multiplicity of the directions of space, the cycles of time, and all the entities and phenomena that exist and occur within space and time. In the human being, in particular, the postcelestial Breath is the breath (qi) of ordinary breathing. In any of its forms, however, the postceles- tial Breath hides and preserves the precelestial Breath, or one “particle” of it. In the strict sense of the term, the purpose of Neidan is the recovery of the precelestial Breath—represented as the Elixir—and its reconjunction with the postcelestial Breath.

2. The first part of this sentence alludes to the description of the center of the human body in the Huangting jing (Scripture of the Yellow Court): “Above is the Hun Numen, below is the Origin of the Barrier; on the left is the Minor Yang, on the right is the Great Yin; behind is the Secret Door, in front is the Gate of Life” (“Inner” version, poem 2). The Secret Door (mihu) is the kidneys, or a point in their region. The Gate of Life (shengmen) is the lower Cinnabar Field, or a point in its region. — The Huangting jing, originally dating from the second or the third century, is one of the main texts on early Taoist meditation. It exists in two versions, usually referred to as “Outer” and “Inner.” The “Inner” version” is later and longer compared to the “Outer” version.

3. This passage is quoted, without attribution, in Xiao Tingzhi’s (fl. 1260–64) Jindan wenda (Questions and Answers on the Golden Elixir). It is also found in Li Daochun’s (fl. 1288–92) Zhonghe ji (Anthology of Central Harmony), ch. 4.

4. This expression derives from the passage of the Daode jing (Book of the Way and Its Virtue) quoted at the end of the commentary to the present section.

5. This expression derives from the passage of the Book of Changes quoted at the end of the commentary to the present section.

6. Daode jing, sec. 6.

7. Book of Changes (Yijing), “Wenyan” (Explanation of the Sen- tences) on the hexagram Kun ䷁ (see Wilhelm, I Ching or Book of Changes, p. 395). The first sentence is also found in the Cantong qi (The Seal of the Unity of the Three), sec. 19: “From the Yellow Center it gradually spreads through the veining: moistening and impregnating, it reaches the flesh and the skin” (see Pregadio, The Seal of the Unity of the Three, p. 77). In the explication given by Wang Jie, these passages of the Daode jing and the Book of Changes refer to the precelestial Breath.

Reference: Commentary on the Mirror for Compounding the Medicine (Ruyao jing zhujie) translated by Fabrizio Pregadio