Metal, as the immaterial Golden Elixir, the Great Medicine of the Mysterious Valley, The Gate or Xuanpin

the Yellow Sprout (huangya), a term that connotes both the essence of Metal (True Yang) found within Water (Yin), and the first intimation of the birth of the Elixir (denoted as "yellow" for its association with Soil, the agent that represents the center). Analogously, lead is black outside, but harbors the white and luminous Golden Flower (jinhua) within.

Cantong Qi

So rereading the book Foundations of Internal Alchemy by Wang Mu - there are a few things that stand out. For example I didn't know that the word Flower is a homonym for the word Transformation in Chinese.

In terms of Metal - the Lungs, as I detailed before, bring the energy down, as the external method of deep reverse breathing - the yin qi goes down while the yang qi goes up. But the Yuan Qi is not created until the yang qi rises to the brain - and thereby turns into the True Lead, as the true Metal (of the lungs).

White is the essence of Metal, Black the foundation of Water. For several commentators, "white" stands for True Lead; for others, it means either mercury, or silver, or gold. These varying views reflect different configurations of the alchemical emblems, in whose contexts the same principle can be represented by different terms and symbols. "White," in all cases, alludes to the authentic principle contained within the "black." Being the True Yang within Yin, this authentic principle is the opposite of black lead, and therefore is called True Lead. For the same reason, it may be called "mercury," which in Chinese alchemy stands in a polar relation to lead. Because of its white color, it can also be called "silver," in contrast to black native lead. Finally, since True Yang is the precelestial One Breath, it may be called "gold," the metal that more than any other represents the Elixir.

p. 74:

the Essence of Metal rises above, and transforms itself into True Lead.

And so the key to this is in the brain the "yang qi" with substance is transmuted into yuan qi - and then descends again with the "yin qi" - down the front, thereby "purifying" the yang qi, into yuan qi.

although the Yang Essence is already abundant, it is still a material entity and thus cannot form the Medicine.

p. 79

So the Essence has to keep being transformed until

it exhausts its firm material Breath.

After Breath reaches the Muddy Pellet, one "withdraws by the Yin response" and the firm breath is transmuted by means of the True Yang Breath.

So again this confirms the Yuan Qi - from the heart to the pineal gland - transforms the yang qi

"into the yielding...when  Yang culminates, it should be nourished by means of the Yin."

p. 80

And the final "barrier" is often described as turning Qi into Shen - and so people think that Shen is more "advanced" than Qi - but this is not accurate.

Essentially, in the formation of the Great Elixir, fire the Spirit enters the Breath [qi], then the Breath is embraced by the Spirit.

 p. 101

Refining the Medicine by means of Fire in order to form the Elixir is like Spirit driving Breath in order to attest to the Dao.

p. 77

When Breath is transmuted and returns to being comprised within Spirit, and thus to being one with Spirit, it forms the Embryo of Sainthood, which is the Elixir. This is the process of entering from Being into Non-Being."

p. 70

Yin and Yang always attend upon one another. Their coming and going is called "transmutation." The inchoate Spirit is at their center, silent and unmoving.

p. 71

So what this actually refers to, has been discovered in quantum relativity - this is precisely the Ether as the relativistic mass of light aka the "hidden momentum" of light - it is also called the "invariant" of light as the noncommutative spacetime of light - from the future. It is what guides light from the future.

"The whole process is ruled by Spirit....It's movement in 'non-doing' is called Original Spirit." Wang Mu, Foundations of Internal Alchemy.

The secret "hidden momentum" of light is the Yuan Qi of the Yuan Shen, the "movement in non-doing. So the light remains unmoving and yet the Breath or Qi continues to transform the energy.