Tugging your meat until you ejaculate, and then pissing. This supposedly cleans your urethra (hence, Cleaning the Pipes)
It stings, and that's evidence that it's doing what its name says.
It can also be done right before having sexual intercourse, so you don't get a woman pregnant, and you don't have to wear a condom.
John: "Ugh, I really need to clean the pipes"
John is Cleaning the Pipes in Cindy's bathroom so he can smash her, rawdog.