Some people are very curious about the yab-yum practice in tantra. Sutra does not have a so-called yab-yum practice; if anything, sutra explains the union of merit and wisdom, not the union of a male and female. The yab-yum practice in tantra is a practice involving the winds, channels, and essences of the body. However, to most people, it is not a practice, but a symbolic representation. For instance, the male deity represents clear light, an aspect of phenomena; the female deity represents emptiness; the union of a male and female signifies the inseparability of phenomena and emptiness. The Heart Sutra says “form is emptiness”—form can denote all male buddhas or bodhisattvas; it also says “emptiness is form”—emptiness can denote all female buddhas or bodhisattvas. “Form is not separate from emptiness, emptiness is not separate from form” represents the union of male and female. People should understand the yab-yum practice from this standpoint.
~Depicted from GATEWAY TO VAJRAYANA PATH - Sutra and Tantra: Similarities and Differences