When you poop and pee and jerk off att the same time.
In order to this ultimate type of masturbation, you'll have to pee a little bit so it's in your foreskin. When you have done this you will have to get an erection and jerk off with the pee inside of your foreskin. When you are about to reales one of the greatest feelings in the world (aka cum) you will have to start pooping.
If you succed you will have made a great achievement called the SBRUNK.