https://en.wikipedia.org/wiki/Vajroli_mudra

Vajroli mudra

This exercise is only briefly explained in the pradipika with its general purpose of containing the vital fluids or exchanging them with a partner of opposite sex. (This has been posted as part of an incel culture screencap about "copulin", vaginal fluids said to penetrate the male organ if the woman is on top during intercourse, which is said to lead to her having mind control abilities over her partner.)

After testing it based on general instructions, I think it can be reduced to saying that it's the contraction of the muscles stretching from the genitals over the front of the pelvic bone to the navel.

When performed in the manner of the bandas, a strong effect can be achieved. It can be integrated with other poses, like the version of the "beetle" from the pradipika, where the feet are held at the toes and pulled as if aiming to put them behind the neck. Other uses are in combination with the "boat" pose.

It is possible to use this exercise to reach the highest plane of awareness and gong development. At this point, the control necessary for prolonged tantric practice can be achieved, that means sustained sexual arousal.