I had a girl " starfish" off a horse this summer, she was barebacking and just dropped her reigns and threw her hands up in the air, half jumped half fell of the horse. Thankfully it was a lazy horse, so he was like "ooohh! Humans gone, I'm done." and stopped looking over his shoulder like "ummm excuse me human, but what are you doing down there? I don't think that's how you dismount."
Equestrianism; 10% luck, 20% skill, 15% concentrated power of will, 5% pleasure, 50% pain and 100% reason to remember you're absolutely insane to be riding a beast that big.