Masturbatingin a way that where your hand slaps and or slapping your scrotum while stroking down to make a smacking noise.
My friend and I had a fapturbating contest we wanted to see who could do it louder before teachers outside the bathroom could hear us. When we fapturbate our scrotums get so red from the smacking.
When you have to masturbate based purely on your imagination, with no porn of any kind. Often used by 12 year olds in dark rooms who have no access to porn, a raging boner, and only the searing memory of seeing up a girl's skirt in school that day.