Going Down

Going Down refers to performing oral sex on your partner: One of the partners has to move his head down in order to reach the genitals of the other partner with her or his mouth

A girl can go down on a guy, which means she gives him a blowjob.
A guy can go down on a girl, which means she starts eating pussy.

Her going down on him	Her going down on him	Him going down on her
Her going down on her	Both going down