The Daoist classic Tai Yi Jin Hua Zong Zhi (known as the "Secret of the Golden Flower" (SOGF) in the west) was initially based on the lyrics of "Xiao Yao Jue" (Ode to True Freedom) and "Quan Shi Ge" (Song of Persuasion) by Lu Dongbin, it began to be circulated in the late Song Dynasty. Later disciples who obtained the manuscript had edited the content over time and it was being transmitted in hand-copied manuscript format. During the Kang Xi period of the Qing Dynasty, it was disclosed and published for the first time by Jingming Pai and Longmen Pai separately. While it was passed down from generation to generation, new contents were added, these contents include fantastical materials obtained from divination and references from Confucius and Buddhist ideas to reflect the influences of these sects in the Chinese society. All these materials had no doubt put the SOGF under a veil of heavy mysticism.
The spiritual world of human being can be divided into three levels: Shi Shen (Conscious Spirit), Yuan Shen (Original Spirit) and Ling Xing (Soul Spirit). Ling Xing came from the Heaven and Earth, Yuan Shen came from your parent, Shi Shen came from the post-heaven world. The relationship between these three levels of spirits is that: Ling Xing is surrounded by Yuan Shen which in turn is surrounded by Shi Shen. Under normal circumstances, when one is awake, Shi Shen is active and Yuan Shen is dormant. When one is asleep, Shi Shen is dormant and Yuan Shen is active. Shi Shen and Yuan Shen does not communicate with each other directly. Ling Xing is the root of life, it's only active at the moment the human life is created or at the moment when the life is ended.
After Lu Dongbin had completed the cultivation as outlined in Ling Bao Bi Fa, he had understood the nature of Shi Shen, Yuan Shen and Ling Xing. Looking from such a high level plane of existence, Ancestor Lu described several specialized topics dealing with Ling Xing and its usages in Ling Bao Bi Fa, the result was SOGF. That is why SOGF is a part of Ling Bao Bi Fa, it described the portions of Ling Bao Bi Fa that dealt with "Tai Yi" (Ultimate Unity). So what is ultimate unity? What can it do? What can it accomplished?
What is "Ultimate Unity" ? Ultimate Unity is also known as the "Ultimate One", i.e. the "one" as in "Dao gave birth to one" in Daodejing. Before there was human being, there was the "one", after human being has disappeared, the "one" will still be aound. To human being, "one" is absolute, eternal and immutable. When a human life is created, "one" enters the body, it's called Ling Xing. Ordinary people do not know about it, whereas the immortal uses it, it's also called the Wondrous Gate, the Ancestral Chamber and the Heaven Heart, it has many names but they are actually all "one". When a human dies, "one" will leave the body.
What can "Ultimate unity" do? "Dao gave birth to one, one gave birth to two, two gave birth three and three gave birth to the ten thousand things". Human being is part of the ten thousand things, if human can understand "one", then "two", "three" and "ten thousand things" can all be easily understood. In Ling Bao Bi Fa, "Ultimate Unity" first appeared in the first segment "Tai Yi Han Zhen Qi" (The ultimate one containing the true qi) of the second method "Collection and Dispersion of Water and Fire". "Jin Hua" (Golden Flower) means the golden cinnabar, i.e. the third segment "The Golden Purple Cinnabar" in the eight method "Return to the Origin to Refine Qi". Therefore SOGF is the principles behind the methods from "Tai Yi Han Zhen Qi" to "The Golden Purple Cinnabar", the ultimate unity can also be added to the various methods between these two levels. In other words, one must at least attain the "Ultimate Unity" in the second method "Dispersion and Collection of Water and Fire", and then utilize it to connect the various levels and segments in Ling Bao Bi Fa in order to finally attain the Golden Cinnabar.
One can imagine that qi is energy, it can't be seen not touched. By using the conscious mind, how much can one condense the qi? "Tai Yi Han Zhen Qi" means that using the "Ultimate Unity" to condense qi, only by this method can one continuously condense qi to higher and higher level of density, from "gaseous" state to "liquid" state and finally to "solid" state of the cinnabar. These states correspond to the levels "Human Immortal Method", "Terrestrial Immortal Method" and "Celestial Immortal Method" in Ling Bao Bi Fa.
What can "ultimate unity" accomplish? To put it simply, the verifications in DanDao are all "seen" using the ultimate unity. Looking it from another way, the ancient Chinese Civilization is based on the objective reality from what is "seen" using the ultimate unity: for examples, the "Yellow Emperor Classic" is the classifications based on what has been "seen", the "Zhou Yi" (I Ching version in Zhou dynasty) is the generalization of what has been "seen". Daodejing is the refinement of the conclusion from what has been "seen". SOGF is the method to practice how to be able to "see".
SOGF originated from Ling Bao Bi Fa, the two classics complements each other and fuse into one, using first level of qi to utilize the Shi Shen, the second level of qi to utilize the Yuan Shen and the third level of qi to utilize the Ling Xing. Through cultivation of Shi Shen one can then stimulate and control Yuan Shen and Ling Xing, which can then leads to the complete understanding of the spiritual world. Obviously, SOGF cannot reveal everything about the "Tai Yi", but if one can understand a little piece of it, one should know to be satisfied.