Fire, lights, sun, moon are usual symbols and seen by most in sadhana. They indicate
movement or action of inner forces. One sometimes sees the Light in masses, sometimes in forms.
The consciousness is usually imprisoned in the body, centralised in the brain and heart and navel centres (mental, emotional, sensational); when you feel it or something of it go up and take its station above the head, that is the liberation of the imprisoned consciousness from the body-formula. It is the mental in you that goes up there, gets into touch with something higher than the ordinary mind and from there puts the higher mental will on the rest for transformation. The trembling and the heat come from a resistance, an absence of habituation in the body and the vital to this demand and to this liberation. When the mental consciousness can take its stand permanently or at will above like this, then this first liberation becomes accomplished (siddha). From there the mental being can open freely to the higher planes or to the cosmic existence and its forces and can also act with greater liberty and power on the lower nature.