Vijnana Bhairava Tantra · Dharana 27 · Verse 50
Where the in-breath ends
Follow the in-breath only as far as it travels, and notice the place in the body where it comes to an end. That inner ending place is the practice, not the nostrils and not the full cycle of breathing. Rest attention where the inhalation stops, and remain there. When the next breath begins, follow it again to the same place.
Older line
When the mind dissolves completely at the far point of the breath within one’s own body, then for one whose understanding is steady, the goal, reality itself, comes steadily into view.