Vijnana Bhairava Tantra · Dharana 41 · Verse 64
Inner or outer junction of breath
Choose one meeting place of the breath and stay there. Rest either at the inner end, where the inhalation dies away in the body, or at the outer end, where the exhalation dies away in the air. Do not track both in this sitting. One door is enough. Keep returning to that single junction until attention stops wandering off along the rest of the breath.
Older line
Through the meeting of the two breaths, within or without, the practitioner at last becomes a vessel for the dawning of equality.