In the gay male subculture, this refers to anal
intercourse where there has been a deposit of semen into the rectum of the partner(s) and the depositor continues to thrust in-and-out of said rectums. Copulatory movements like the primal reflex of pelvic thrusting is the prime action causing the situation. After
ejaculation in to whatever receptacle, the male of most vertebrate species stops penetrative motions (if the species needs it) and returns to nominal. However, when semen is deposited into an environment not intended to receive and chemically balance it (like a rectum), the result of this prolonged thrusting is an aeration of the proteinaceous materials abundant in human semen, the majority of it volume being water and it's pH being comparatively basic, this leads to easy spoilage. Thus the resulting product is does not remain the opalescent milky water mixture, but rather is changed into a frothy, opaquely white material. This is where the terminology
originated when used in the aforementioned context: gay males often prolong their gyrations to assist their partner in achieving climax whilst together.
For that matter, it can of course happen in male-on-female
intercourse with the female's rectum being the receptacle, but the
association is not widely maintained.