I did a little cowboyin’ in my younger days. I would be careful with that one rein stop, it can be dangerous itself. Although I don’t think the best advice in the world can help a scared and inexperienced person on a runaway horse too much (they're just too freaked), I would recommend RIDING the horse, hang on as best you can (saddle horn, mane) and stay loose (don’t stiffen your muscles). When the beast gets home or far enough away from what scared it will stop. And of course the screamin’ part doesn’t help much either.