The first is an extension of regular energetic chi-kung awareness. The intensity of sex catapults ones practice farther, as channels and chakras are exercised using much more intense energy. One doesn’t merely raise kundalini during orgasm, but raises it as the act of sex. One also lowers energies and moves them all about. Kundalini is sex. Orgasm is optional, and usually avoided.
The second aspect is devotional. The beloved and and oneself unite, and in the communion there is a respect for our essence. This is becoming who we really are, which is prayer.
The third aspect is non-dual. Not just a union, but beyond time or space or any possibility of separation. Without this third, really we should not use the name “Tantric”. We should call it Taoist, or energetic, or chi-kung sex. Tantra has the notion that kleshas and perceptions can arise as self liberated. It is extremely subtle, and usually only approached after many years of intense meditative and philosophical engagement. Merely raising kundalini and not coming does not tantra make.