Here's Wikipedia on Tantric sex:
When enacted as enjoined by the tantras the ritual culminates in a sublime experience of infinite awareness, by both participants. The Tantric texts specify that sex has three distinct and separate purposes — procreation, pleasure and liberation. Those seeking liberation eschew frictional orgasm for a higher form of ecstasy, as the couple participating in the ritual, lock in a static embrace. Several sexual rituals are recommended and practised. These involve elaborate and meticulous preparatory and purificatory rites. The act balances energies coursing within the pranic ida and pingala channels in the subtle bodies of both participants. The sushumna nadi is awakened and kundalini rises upwards within it. This eventually culminates in samadhi wherein the respective individualities of each of the participants are completely dissolved in the unity of cosmic consciousness. Tantrics understand the act on multiple levels. The male and female participants are conjoined physically and represent Shiva and Shakti, the male and female principles. Beyond the physical, a subtle fusion of Shiva and Shakti energies takes place resulting in a united energy field. On an individual level, each participant experiences a fusion of their own Shiva and Shakti energies.[7][13]
In Vajrayana Buddhism the related set of practices are known as Karmamudra, the 'Action Seal', and is only sometimes included in the Six yogas of Naropa. Due to monastic vows it is generally only visualized. The maithuna is called yab-yum in Tibetan. Sushumna, is perceived as blue and called Avadhuti in Sanskrit. It is said to carry the wisdom prana (Yeshe or Rigpa in Tibetan). The karmic channels, ida and pingala, are respectively the white right channel, Rasana, and the red left, Lalana. Some sources affirm that they are the converse for women, some non-heterosexual and gender-fluid individuals.[14]