When having intercourse with a girl doggy style(can happen during vaginal or anal sex), there is another girl under her on her back in 69 position, with her mouth positioned right under the point of penetration. As the male ejaculates he does so onto the girl's ass crack and it drips down into the other girl's mouth and she swallows all of it. Or she can continue to snowball into the other girl's mouth.
e.g. usually happens when a man pulls his penis out of a girl's anus or vagina and ejaculates onto her ass crack,so that the girl below on her back can eat the cum.Hence the term white wash waterfall.