Yin and Yang

For the firing process, the classics speak of the I Ching hexagrams;
But it is wrong to ponder the lines and cling to the images.
It is all a matter of needing to understand the principles of yin and yang; Advance and withdraw according to the time, as is indicated.
-- Liu Yiming (18th c. Taoist master, The Inner Teachings of Taoism)

Yin and Yang are the building blocks of the trigrams and hexagrams.

There is not only one kind of yin and yang. There is primordial yin and yang, and there is condition yin and yang. There is yin and yang within life and there's yin and yang within essence. There is real yin and yang and there is artificial yin and yang. There is external yin and yang and there is internal yin and yang. It's necessary to study all these kinds of yin and yang and understand them clearly before starting the alchemical work. -- Liu Yiming (Commentary on Understanding Reality by Zhang Boduan)
The mind of Tao is associated with celestial yang, in contrast to the human mind, or human mentality, which is associated with mundane yin. -- Liu Yiming (The Taoist I Ching)

Mundane yin refers to the illusory world created by the human mind. It can also be called the world of the lower self and is characterized by the state of imagination (uncontrolled mind activity), illusionary suffering, undue preoccupation with trifling matters and mechanical habits. This illusory world obstructs the mind of Tao.

The firmness of the Tao belongs to yang; this is the “great”. The flexibility of the human mind belongs to yin; this is the “small”. If the human mind lacks the mind of Tao, it can defeat the Tao by using consciousness to produce illusion. If you govern it by the mind of Tao, the conscious light is clear and can thereby help the Tao.
-- Liu Yiming (Commentary on Understanding Reality)
The method used by the ancients for escaping from the world consisted in refining away the dregs of yin in order to return to the pure Qian (the trigram ☰ or hexagram䷀ of Heaven). It is just a matter of dissolving the lower self and completing the higher self.
-- The Secret of the Golden Flower

This yin energy is artificial yin and is often symbolized by animals and demons.

When the spirit of heaven rules in man, his animal nature takes its appropriate place. -- I Ching (Commentary on Hexagram #11 Peace, Wilhelm/Baynes edition),
“Bodhisattva,” said Monkey, “he's the blue-haired lion from under your throne.” The Bodhisattva said a spell and shouted, “Return to the Truth, beast. What are you waiting for?” Only then did the fiend-king return to his original form, Manjusri placed a lotus-blossom over the monster to tame him, and sat on his back. -- Journey to the West Ch. 29
The body of pure yang was happy in the sun, the yin demons dared not use their might. -- Journey to the West Ch99

Celestial or true yang energy refers to the pure conscious awareness of the Original Spirit or Higher Self. It is sometimes accompanied by a feeling of wonder and gratefulness to simply be alive. Experiencing this awareness is the inner meaning of all esoteric traditions and a myriad of symbols are used to describe this state of the presence of the Original Spirit.

When the primordial has not been injured and true yang is within, the single mass of true energy is ‘the superior man’. When the primordial has been lost in the true yang is scattered internally, whatever the body has is false energy. -- Liu Yiming (Taoist I Ching, Hexagram #12)
These are examples of genuine arising of yang. It may happen that while you are reading books or reciting poetry, personal desires suddenly vanish and a unified awareness is alone present ‑ this is one aspect of the arising of yang. Also, sometimes when friends gather and talk, they reach a communion of the inner mind, and suddenly yang energy soars up and the true potential bursts forth ‑ is also one way in which yang arises. -- Huang Yuan-ch'i (Annals of the Hall of Blissful Development) as quoted in The Taoist I Ching
The firmness of yang centrally balanced, the spiritual embryo takes on form; one has "filled the belly." -- Liu Yiming (Commentary on hexagram #3 Difficulty, The Taoist I Ching)

True yang energy very occasionally arises spontaneously. Everybody has experienced it a few times in their lives, but most people don`t know what it is, other than that they felt they had a special experience. To experience this energy at will, a method is needed.

The great Tao is natural and spontaneous, without artifice. Why is it necessary to use a method of deliberate action? The reason it is indeed necessary to use the method is to get rid of degeneracy. When all degenerations are effectively done away with, then the method is not needed, just as a raft needed to cross a river is to be left behind once the river is crossed and a net needed to catch fish is to be put away once the fish has been caught. -- Liu Yiming (Awakening the Tao)

As was said in the introduction, one of the many names of this method is called the firing process.

The firing process spoken of in the alchemical classics and writings of the masters is a metaphor for the order of practical spiritual work.
-- Liu Yiming (The Inner Teachings of Taoism
When mundane (yin) is stripped away, and the celestial (yang) is pure, the firing is sufficient.-- Li Daochun (13th c. Taoist master, The book of Balance and Harmony)

The first six images of the ten oxherding pictures represent the firing process through which the human mind (false yin) is stripped away by the firmness of the mind of Tao and yang energy becomes pure.

The mind of Tao is the primordial jewel of reality, it is produced from within the temporal; there is a time to use it, and a time not to use it. Before you have crystallized the elixir, you need to use the real perception of the mind of Tao to govern the conscious perception of the human mind. After the human mind is quiet, and discriminatory awareness is extinguished, the mind of Tao then has no function.
-- Liu Yiming (Commentary on Understanding Reality
The point where one yang begins to move is when the yang light of real knowledge of the mind of Tao stirs but is not yet very active. At this time you should quickly set about increasing the fire, gathering yang, gradually refining, from vagueness to clarity, from one yang to complete purity of six yangs. This is also like the mid-autumn moon, exceptionally bright, shining through the universe. -- Liu Yiming (commentary on Understanding Reality by Zhang Boduan))
This hexagram (hexagram #44 ䷫ Meeting) represents warding off mundanity and preserving celestial energy. Adding from one yang (hexagram #24 return) until there are six yangs (hexagram #1 Heaven) , so that finally positive celestial yang is pure and complete.
-- Liu Yiming, The Taoist I Ching, hexagram #44 Meeting

The last four pictures represent pure or true yin energy.

The 2nd century text Cantong Qi (Triplex Unity) speaks about the time cycles of the sun and the moon representing the firing process and matches them with certain trigrams and hexagrams.

The inner development of the elixir and the timing of it`s firing are no different at all from the ebbing and the flowing of the phases of the moon. -- Yu Yan (13th c. Taoist master, Commentary on the Cantong Qi)
When the full moon of contemplation is reached, you will be pure.
-- Journey to the West Ch. 19

Yin and yang also refer to inhalation and exhalation.

The rising and descending of exhalation and inhalation in alternate succession corresponds to the relationship of yin and yang. The Cultivation of Realization (by an unknown author, discovered and published in 1739)
It is a great matter indeed that man has his breath! There is a saying: As you breathe, so the firm and yielding rub against each other, so they form the very image of Qian (Heaven) and Kun (Earth), opening and closing. --Yu Yan (Commentary on the Cantong Qi)
Mysterious and remote are the two breaths,
yet their influence pervades all things,
How much more so, close by, in your own body,
Intimately within your own breast,
Yin and yang match the sun and the moon,
And water and Fire attest to it.
-- Wei Boyang (Canton Qi)

Both Heaven and Earth, and Fire and Water, can refer to the inhalation and exhalation. The Yin and Yang in Fire and Water is not pure, while Heaven and Earth are pure Yin and Yang. Fire and Water are the medicine that bring one back to the True Breath, while Heaven and Earth are the True Breath.

Yin energy - animals representing thoughts going round and round in the human mind (Click to enlarge)

Tarot card X, the Wheel of fortune

Manjusri Bodhissatva seated on his lion.
The lion represents the lower self, Manjusri represents the Higher Self or Original Spirit; Beauty and the Beast.

Thai classical dance
(Click to enlarge)

The laughing Buddha

Ten Oxherding pictures
(Tomikichiro Tokuriki)

Hexagram 1# Heaven

The six trigrams associated with the 30-day moon cycle according to Triplex Unity.
(Click to enlarge)

God the Father (representing Heaven) giving the Breath of Life to Christ (representing
Earth) and breathing the spirit into him.
They have the same face, symbolizing
that this occurs within a person
who performs the firing process
(Click to enlarge)