Hands free cum..how i do it

i am asked this a lot so here goes. when taking a pee and you want to stop the flow half way through you tend to "pull" the base of your cock shaft in, it's kinda like twitching your cock i guess, well that's the same pull i use for a hands free cum but i "hold in that pull" for as long as it takes to erupt, plus also you must stay super hard on stainless steel cock ring and be REALLY wanting to cum for it to work. why not give it a go, it feels incredible when that cock starts spasming and spraying cum all by itself :)