Headache is not a sign of the force descending, it is only a result sometimes of some difficulty in receiving it. If there is no difficulty in receiving, there is no headache. The signs of the force coming are the pressure to be quiet, the sense of peace coming or wanting to come and many others, such as a feeling in the head or body of something coming in like a stream or a current or shower etc.

Ref: Letters on Yoga – III