Vijnana Bhairava Tantra · Dharana 19 · Verse 42
AUM thinning to the cut-point
Sound a seed syllable such as AUM aloud, and let it fade on purpose. Place attention on the very instant the sound becomes nothing. Do not settle for the general quiet afterward. The practice is that exact moment of ending.
Older line
Sound a seed syllable through each of its letters, and then through its ever finer resonances, the half-moon, the point, the vibration, and its ending, until it is uttered as emptiness. So one becomes Shiva.