Yidams have both male and female forms. The male wrathful yidam is known as heruka which means “blood drinker,” he who drinks the blood of ego. The female wrathful yidam is called a dakini. The dakinis are tricky and playful. The male and female of the peaceful yidams are known as bhagavat and bhagavati meaning “glorious one.”

The male figures signify awakened energy, skillful means, bliss. The female aspect is compassion, emptiness, and intellect (which, as the emptying of confusion, is passive rather than active). The emptiness signifies fundamental accommodation and also ultimate fertility in the sense that emptiness is the mother of form. Through union with the heruka, the dakini can give birth to enlightenment. The dakinis in general reinforce the nature of their consorts and the bhagavati has the role of asking the bhagavat on behalf of all sentient beings to proclaim the teachings.

In general the union of the male and female aspects, known as the yab-yum (“father-mother”) form, is a symbol that skillful action is impossible without compassion, that energy cannot be effective without intellect, and that bliss is impossible without emptiness. This symbolism denotes the interaction of these elements as aspects of enlightenment, rather than on the ordinary confused level of indulgence in passion and aggression.

— Chögyam Trungpa, The Collected Works of Chögyam Trungpa Rinpoche: Volume 7