The silence is the silence of the inner consciousness and it is in that silence unmoved by outward things that the true activity of the consciousness can come without disturbing the silence — true perceptions, will, feelings, action. There also one can feel more easily the Mother’s working. As for the heat, it must be the heat of Agni, the fire of purification and tapasya; it often feels like that when the inner work is going on.

Ref: Letters on Yoga – II