used to do that as well when really horny, jack off a load, tie off/pinch my foreskin, store the load of cum...then use it to jack off again, or once soft, self fuck....
have tried the holding the piss back in the shower, interesting but doesn't do a lot for me...worse thing for me is having to piss BAD and not getting the foreskin back in time and turned into a 'sprinkler' (anyone else have uncut sons? damn could they make a mess....one was notorious for not 'making it' in time....finally told him to pee in the shower lol