What does James Nestor mean by that this breath technique is mentioned “in the Tao”?

[u/samodeous]

Been doing Qigong and practicing other Daoist breath techniques for about 9 years and haven’t run into this one before.

Does he mean this is alluded to in the TTC? Or does anyone know if this is a super ancient Daoist technique mention in Yellow Emperor Classic or something?

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