Why Freezing the Fire as Spirit (Fire) is also the Double Reversion secret of alchemy

3 hours ago, Bindi said:

I don't believe Daoist alchemy is the same as kundalini yoga at all. Visualising fire at the base of the spine sounds like kundalini yoga to me. 

yes although this can also be called "Freezing the Spirit" - in the Daoist Yoga: alchemy and immortality book. It states - if you don't understand this book at all - just do "freezing the spirit" and that will take care of all the rest of the details.
https://archive.org/stream/TaoistYogaAlchemyAndImmortality/Taoist-Yoga-Alchemy-and-Immortality_djvu.txt

so the secret is that this is the "double reversion" as Pregadio calls it. In other words - you have the fire go down (visualize into the Earth). Then the water rises up and turns into Air. But then the water has to go down - the FRONT - as now transformed - and this is the "freezing of the spirit."

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freezing the fire...fire derived from spirit.

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‘Freezing the fire’ is freezing and driving spirit into the cavity of vitality (under the navel)....‘All masters taught only the freezing (or fixing) of spirit in the cavity of vitality.*

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he should first freeze spirit to return it to primeval darkness until in this condition of utter stillness it radiates and illumi¬ nes the whole of empty space

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It is the consolidating method which consists of turning back the seeing and hearing to empty the heart (of sense data) so as to freeze spirit and so gather vitality which will then stop draining away.

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When advancing further in the training, if the practiser, instead of cultivating (eternal) life, merely closes his eyes to freeze his heart and so disengages all six sense organs, thereby cutting all links between the front and back as well as the upper and lower parts of the body, how can he develop prenatal vitality?

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In the last chapter we explained the method of freezing spirit which is so called because it consists of first gathering prenatal true vitality [Yuan Qi] in the original cavity of spirit (tsu ch’iao in the centre of the brain) and then driving it into the lower tan t’ien (under the navel).

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By stoppage of respiration is meant the condition of serenity in which the practiser becomes unconscious, his breathing (almost) ceases and his pulses (all but) stop beating. This is called freezing spirit.

So that is what Sri Yukteswar called "Breathless Ecstasy"

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Freezing spirit: An alchemical process which ensures the 

condition 

of serenity in 
which the practiser becomes unconscious, 
his breathing almost ceases and his pulses seem to stop beating 
for the purpose of gathering prenatal true vitality in the original 
cavity of spirit in the centre of the brain and then driving it 
into the lower tan t’ien under the navel to hold it there to 

achieve 

immortal breathing.