A Latin term for the practice of sexual intercourse while maintaining chastity, between husband and wife in a “spiritual marriage.” The women who practiced this were referred to as the virgins subintroductae. The term “subintroduction” refers to the special way of uniting the male and female sexual organs:
During the Middle Ages, many Gnostics practiced the Maithuna with Vestal Virgins, calling this marvelous practice Virgine Subintroductis (Sexual Magic). The Virgine Subintroductis with Vestal Virgins was formidable. It was practiced in the form of Karezza, so that the vestals retained their virginity. In this excellent practice, the man and the priestess lay on their sides, making sexual contact. The man introduced his phallus gradually, with extreme caution, between the vaginal labia and the hymen. With time the hymen became elastic, thus enabling deeper penetrations each time. Thus, this is how vestals never lost their virginity; they remained virgins for their entire life. The man attained realization raising Kundalini through the spinal canal with a virgin vestal. —Samael Aun Weor, Light from Darkness
See: syneisaktism, What is sexual magic?, tantra