Excellent quote from Pregadio on how the Essence of Heaven is contained in Water

In this view, the cosmos as we know it is the self-manifestation of the
Dao, which first determines itself as Oneness. Primal Oneness contains Yin and Yang in their pristine state; the joining of Yin and Yang generates the world. To appreciate the details of this view, it is convenient to follow the example of the alchemical texts and describe it through the emblems of the Book of Changes. Qian (pristine Yang) and Kun (pristine Yin) are constantly joined to one another in the state of Unity. Being joined Qian unceasingly bestows its essence to Kun, and Kun bri ngs it to achievement; thus the world with its countless events and phenomena is generated. However, due to the very fact of being continuously joined to one another, Qian becomes Li (Yang} , and Kun becomes Kan (Yin). Therefore the essence of the Yang principle in its pure state is now found within Kan. That principle, which is the One Breath of the Dao (the state of Unity represented by the undivided line), is what an alchemist seeks to recover.

 Awakening the Tao


 In The Way of the Golden Elixir, Pregadio gives a pretty decent explanation.

According to one of the ways in which this passage has been understood, the Dao first generates Unity, which comprises the two complementary principles of Yin and Yang. After Yin and Yang differentiate from one another, they again conjoin and generate the “three,” which is the product of their conjunction. The “ten thousand things” are the totality of entities produced by the continuous reiteration of this process.

 So now we switch over to Pregadio again - and discover this double reversal secret from the rotating of the eyes - the True Yin is the yang shen plus the yang qi (that goes down the front as the White Tiger True Yin). The True Yang is the yin shen plus the yin qi - that is from the yin qi going up the back. So the yin qi originates on the front as part of the yang shen and the yang qi originates on the back as part of the yin shen. But to create True Yin (Earth) and True Yang (Heaven) then this double reversal has to occur.

Normally the yin qi is is lost out of the eyes. So with meditation the yang shen is closed off with the eyes closed - and then the yin qi goes down. But then it joins into the yang qi and has to then go back up again on the back (as the double reversal as the True Yang or Heaven). Then the opposite is true for the yang qi - normally is originates on the back and has to go up but then it has to join with the yin qi to go back down again on the front as the True Yin (or Earth)

 http://elixirfield.blogspot.com/2019/02/the-secret-to-storing-qi-internally.html