An injaculation happens when the semen is transferred back into the body, usually by placing pressure on the perineum.  There are a number of reasons why this practice can be beneficial, sexual energy is believed to assist the body with rejuvenation, raise your spiritual awareness and masters use this method as a way to enlightenment.

Here is a link to a clip from Mantak Chia, a Tao master, similar instruction, he gives full details on how this can be achieved for men and women.

The Big Draw 2 - Mantak Chia