As said I used to do it as a party gag - do you have strong pelvic floor muscles ?
Yes I have strong pelvic floor muscles. I stop/start my pee when I am bored on the toilet. Maybe I should put a few bb mags in there instead?
As well, I believe that lifting weights strengthens them. Because when you are lifting quite heavy (heavy for the individual) you tend to pull everything in. You can play games in the bedroom also, by tensing your pelvic floor muscles around the cock. Regular orgasms also strengthen the pelvic floor muscles.