Often referred to as Mist of Love.
When a boy sees a beautiful girl on the other side of a window, he will masturbate right in front of the glass while staring at the girl continuously. The girl will be totally scared, but only if she sees the guy. Otherwise it will do no harm to her, as she won't notice. The heat caused by the penis will be so strong that the glassed window will start to become foggy over the time.
Brad : Hey, last night I walked outside and I saw this beautiful girl in the gym around here. She was running and sweating and it was so sexy.
Robinson : So, did you go talk to her?
Brad : Even better. I stopped in front of her and started to masturbate.
Robinson : Ahhhh, I see. I believe the window had a lot of fog of love when you did that.
Brad : Yeah, there was a huge circle. But when the girl noticed I was staring at her, she couldn't see I was masturbating because the window had too much fog on it.