This generally hurts quite a bit as it takes the other person by surprise and is known as a precursor to the Nash Splash, though once a Nash Bash has occurred there is little time for the partner to escape before the Splash happens.
Guy 2: Yeeah, it was awesome. She was going down on me and wanted me to warn her when I was going to spurt, so I did, cept then I gave her a nash bash for being cheeky and not swallowing.
Guy 1: Sweet dude.
Yeah, I got completely hammered before the game and broke my leg on the pitch. I can't wait to go back!