Meditate on the supreme and unchanging [bliss].
With vajra [male sexual organ] placed inside lotus [female sexual organ],
brings the winds [vital forces] into the drops [bodhichitta],
the drops into the chakras;
the movement of drops halted at the vajra [male sexual organ],
always rigid, the yogi continually raises fluid [never releasing it].
With the Yoga of mahamudra [great seal],
the descent to the vajra, and by its blessing,
the instances of supreme unchanging [bliss],
completed at 21,600,
will bring the great enlightenment,
the attainment of Vajrasattva [diamond soul].

Quoted from Ornament of Stainless Light, an Exposition of Kalachakra Tantra, by Khedrug Norsang Gyatso (15th century AD).
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