Duomo fire is the vital heat of the gross vital form, the fire in the navel centre. The three finger is referring to the thickness of the subtle flame extending from the base of the spine going up through the top of the head and then down the frontal line. The spine being the conductivity of vital energy. If we allow this conductivity we can intensify our vital energy. So, using electricity as an analogy – instead of operating on 20 watts, we begin to operate on 20,000 watts.
It is a practice of conductivity of the life force to call up the duomo fire and maintain our body heat. There are true stories of monks being able to tolerate very low temperatures, where they have gone out into the snow with a wet sheet over them and they are able to dry the sheet and maintain their body heat. This is an art and such an extreme circumstance is not expected to be practiced, but it is a demonstration of the fire of transformation initiated in the central channel via a yogic practice.