The purification. It is the purification of the physical that is usually indicated in the symbol of burning.
The sadhana is based on the fact that a descent of Forces from the higher planes and an ascent of the lower consciousness to the higher planes is the means of transformation of the lower nature
although naturaly it takes time and the complete transformation can only come by the supramental descent.

Most of the methods aiming at self-control have indeed made use of repression, of the suppression of movements with the idea that if one continues this suppression long enough, one succeeds in killing the element that is not wanted. This would be quite true if it were a question only of the physical world, but behind the physical world there is the subconscious world and behind the subconscious world there lies the immensity of the Inconscient. And what you do not know is this that unless you destroy within you the desire itself, that is, the seed of the formation, this formation which you are preventing from manifesting is so to say repressed in the subconscient driven down and repressed right at the bottom and if you go and search in the subconscient you will find that it is waiting there to do its work.

A finer consciousness with happier lines,
It has a tact our touch cannot attain,
A purity of sense we never feel;
Its intercession with the eternal Ray
Inspires our transcient earth's brief-lived attempts
At beauty and the perfect shape of things.