Another important, although brief, previously unnoticed reference to the Kapalikas is found in the well-known Nath-sampradaya text, Svatmarama’s Hatha-yoga-pradipika. Svatmarama claims to describe the Amaroli mudra “according to the khanda doctrine of the Kapalikas.” The Amaroli mudra is a variant of the better known Vajroli mudra, a sexual yogic exercise in which the adepts reabsorb the fluid released during intercourse.
-Criminal Gods and Demon Devotees: Essays on the Guardians of Popular Hinduism