Marpa returned to India bearing sixty sang of gold. On the way he traversed Nepal, where he met his previous teachers and made offerings. Learning that [his teacher] Naropa had entered the action [a high degree of attainment], he journeyed to the city of Lakshetra in the West and asked for Yeshe Nyingpo who was staying in a forest, attended by a woman of low caste. "When she comes to gather water," he was told, "ask her if you may see him."
When she arrived, he made his request. Pointing to a large jar of water she was holding [in other words: her own sexual organs], she said, "Use one-third of this water to bathe, and one-third for drinking."
After Marpa had finished bathing [his mind through meditation], the washing water fell back into the vase [of sexual union] and transformed into his white seed-essence (bindu; masculine sexual energy; in other words, he did not spill a drop of fluid, and it returned, transformed].
The woman then performed a [Tantric] yogic exercise over the vase [the vessel of sexual union] and from her secret place [yoni] emanated a stream of [energy, the feminine] red seed-essence [not physical but vital energy]. After the two essences had merged [by means of their yogic practice] and Marpa bathed [his mind] in the [sexual] water again, all his preconceived thoughts dissolved. And when he drank the water [through transmutation] and looked into the vase [of sexual union] he saw the full assemblage of the [Tantric] Guhyasamaja deities.
Quoted from "The Great Kagyu Masters: The Golden Lineage Treasury" (Snow Lion, 1990)