Vijnana Bhairava Tantra · Dharana 17 · Verse 40
Before and after a sound
Sound a single syllable, aloud or silently. Attend to the silence just before it begins and the silence just after it ends. Let the syllable become a thin bridge between two emptinesses, and rest at those two edges until you seem to become the silence they share.
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Contemplate the emptiness at the beginning and at the end of any syllable. Through that emptiness, one becomes emptiness itself.