Vijnana Bhairava Tantra · Dharana 55 · Verse 78
One side of the seat, no props
Sit on a soft seat with the weight on one side of the pelvis only, and give the hands and feet nothing to lean on. The slight instability is the method. If attention drifts, the body begins to tip, and that brings you back. You are not seeking a dramatic posture. You are using a mildly uncertain seat so that presence has to stay in the body.
Older line
On a soft seat, resting on one buttock alone, with hands and feet unsupported: staying so, the mind becomes supreme and full.