Vijnana Bhairava Tantra · Dharana 21 · Verse 44
Crown void and base void
Sit with the eyes closed. Hold two regions at the same time: the space above the crown of the head as empty, and the space at the base of the body as empty. Do not alternate between them. The work is to keep both empty together. When attention is held this way, thoughts have fewer places to land, and they begin to thin.
Older line
Whoever holds together the emptiness overhead and the emptiness at the root becomes empty of thought, by a power that needs nothing from the body.