As a teenager climbing a tree in my yard I'd get these incredible feelings and spurt in my shorts by doing pull ups and leg lifts. I actually didn't know what an orgasm was at that time but I soon learned and did a lot of climbing in that tree. I became the pull up champ at my high school, and no one knew the real reason why It's been many years since then. I can't cum that way as easily any more. But from time to time I can still pump out a bunch of loads hands free doing leg lifts and hanging from a chinning bar - like the guy in the X-Tube video. I don't know how I got wired that way, nor do I know how you learn to shoot that way. There's something in holding the tension in my upper body and abs, flexing my abs and trying to channel the feelings into my core that sets it off.