After ejaculating once, cleaning out the pipes is when he keeps fucking or wanking hard into another, weaker but still present orgasm, which doesn't expel any semen except that which was leftover in the penis from the original orgasm. Harder for those older, easier done by those you and testosterone filled.
Cool dude: "Man last night i was with mah girl and i came once, then i cleaned out the pipes and she loved it"
Lame dude: "Yeah, I cleaned out the pipes too, but I was alone..."
Other dude: "I heard about some guy in ST. Louis cleaning out the pipes, i heard he could do it twice in a row!"