Either during sex at orgasm or while your 'one night stand' is asleep.
the idea is you cum and shit all over your partner at the same time giving you and your partner your weird sexual jollys. if your partner is asleep then you do the same process without waking her up and leave her alone in her house/flat, so she wakes up with shit and cum all over her body, make sure you give her a fake name so she cant track you down.
Ex1 - dude i was banging this chick lastnight and she asked me to double jack all over her face!
Ex2 - hey i had this one night stand lastnight and when she was asleep i doublejacked all over her body and i went back home!