Drive Spirit (fire) under the Water as Quick Fire: no it is not "just" sublimation? No it is also purification (down the front back to the lower tan t'ien).

And this quick fire breathing is what causes the alchemical agent to arrive in the original cavity of the spirit (niwan palace), correct?thanks

You have to do the full "Small Universe" meditation - to purify by bringing the yin qi back down.

  Quote

After it has shrunk the practiser should breathe in to drive the generative force into the base of the spine and thence up the backbone to the brain; and out to send it down to the lower tan t’ien centre (under the navel). Quick fire is produced by strong and slow fire by gentle breathing. It is most important that each complete breath should turn the generative force back to the source (below the navel) in order to put an end to the arousal of the penis.

and

  Quote

Hence the method of correct breathing to circulate quick fire up and down (in the microcosmic orbit) and so immobilise the generative force causing the genital organ to retract (thus stopping the drain of vitality). This secret of sublimation consists in one word ‘knowledge’,

So the "purification" is when the yin qi goes back down from the heart - to the lower tan t'ien (along with the yang qi that was brought up the spine as sublimation).
The quick fire activates the "inner cavity" of the tan-t'ien - so that the yang qi has the power to drive out the yin jing blockages (lower emotional blockages).

  Quote

arousing the inner fire so that both outer and inner fires act on each other in order to enlarge all obstructed psychic channels and clear them of obstructions. Since positive fire is stronger than generative force, it drives the latter into all parts of the body to remove all impurities thus preventing the development of ailments.

The source of the Yuan Qi - is "motionless change" aka "noncommutative phase" - meaning it is the "doing in movement" of the Yuan Shen - the light of no light.

  Quote

‘Shifting’ is motionless change, that is transmutation by quick fire which results in nothing any longer having form or shape whether seen closely or from a distance, and includes the elimination of every impure residue. Only this can be called true efficiency.

and eventually then the golden light soars up from the yang qi that accumulates and interacts with the yin qi.

  Quote

as a result of pointed concentration a mass of true inner fire will soar up causing spiritual light in the cauldron to illuminate the whole body the four elements of which (see note 3 on page 41) are thereby sublimated.

So this pointed concentration means the sun is in the moon and vice versa - as the positive fire goes out the eyes when they are open. 2/3rds of the brain's power is used by the eyes when they are open. So in meditation this light is turned around - and the left eye (yin qi) and the right eye (yang qi) then combine in the lower tan t'ien.

pointed concentration in order to drive spirit 
into the cavity of vitality (under the navel).

and 
closed my eyes to make pointed concentration to kindle 
the slow fire in order to lower the (vital) breath in my 
heart to stabilise my spirit and intellect.

After rolling his eyes, he should close them and as his 
mouth is now full of golden nectar, he should make pointed 
concentration on the vitality which he has just circulated 
(as above), driving it down (the jen mo or channel of func¬ 
tion), followed by the golden nectar, into the cavity of 
vitality (below the navel). This is returning vitality to the 
cauldron to seal it there.

‘When the sun and the 
moon without unite (i.e. when the pupils of both eyes are 
drawn close to each other in a squint for pointed concen¬ 
tration), the heart (the house of fire) and lower abdomen 
(the house of water) within are automatically linked; 
only then can prenatal vitality [yuan qi] develop gradually.

And so he should be determined 
firmly to preserve this serenity at all costs by concentrating 
his spirit pointedly on this cavity, which is the most im¬ 
portant thing at this stage of training. 

Even 
before the heart is immersed in sleep, the pupils of both eyes 
should be drawn close to each other for pointed concen¬ 
tration on the great emptiness so that in the condition of 
utter stillness the vital principle returns automatically to its 
source (under the navel), breathing continues normally and so 
is self-regulated, and the (vital) breath is brought under per¬ 
fect control. This method of sleeping will banish all dreams 
and so prevent the generative fluid from draining away.

Then concentrate both eyes pointedly on the lower tan 
t’ien centre (under the navel) and breathe deeply so that 
your breath reaches that centre. If you do this for one hun¬ 
dred days your lower tan t’ien will become hot, your eyes 
will flash light and you will hear something like wind 
blowing in your ears; you will thus restore the positive 
principle that has been dispersed. You should then practise 
microcosmic orbiting (by turning the wheel of the law) to 
repair the damage, doing it gently in order to avoid strain. 
If you so train you will certainly produce the immortal 
seed which, however, before it matures, tends to arouse and 
shake the penis without prior warning. 

‘The method of 
producing the immortal seed consists of “uniting” the heart 
with the eyes for pointed concentration on the lower ab¬ 
domen so that if vibration is felt there, vitality can be 
immediately circulated (in the microcosmic orbit) for 
purification, but the slightest carelessness on the part of the 
practiser can cause it to drain away thereby nullifying 
all previous progress.

So - physics have proven that light has a "hidden momentum" that is from relativistic mass - meaning Emptiness or spacetime curvature. So when spacetime shifts there is a change in the light frequency, due to the wavelength of spacetime shifting. But quantum energy is based on frequency directly. Louis de Broglie realized that there is a limit to relativity - a paradox - that he solved - called the Law of Phase Harmony.
So the yin qi has red light - and the yang qi has blue light. When the Yuan Qi is activated as the Yuan Shen - at first it is yellow-white light - this is called the Full Moon in the Sun - when the yin qi is fully restored. As the yang qi builds up - then it creates red light from the lower frequency energy (the jing) - so that is the Sun within the Moon - the "light of vitality" or yang qi.
So then you get ambrosia - which is swallowed - to return the qi and shen back to the lower tan t'ien.
As the energy builds up - this ambrosia turns into the golden Nectar - as it has more yang qi within it.
It still combines with the yin qi of the five organs. As you deepen the practice the yin qi is converted into more yuan qi, via building up the yang qi. The Yuan Qi is the "light of no light" as the hidden pressure or momentum within the Shen light - so it is also the Yuan Shen manifesting - the Spirit of No Spirit.
So this is why the yin qi light (white) and yang qi light (golden) then create the yuan qi  as the hidden momentum of light from the light being turned around - into the opposites of frequency and wavelength.