What does going down on a man mean in gay porn?

What is going down?

Common expression for someone who sucks on another person's penis. The act of bending, or dropping to one's knees, to perform Felatio on another person.

Definition

Has various additions, such as 'shower head' in which one is given a blowjob, in the shower.

Going down like a submarine, would refer to someone who takes to dropping to their knees, to suck a penis, with desire, with speed.