Naturalhorse got that right. A riding buddy of mine years ago had a mare that if you let her get stretched out in a run was about impossible to stop. We had both already decided anytime we would run our horses, I always stopped mine first then she would work on getting hers stopped and we both knew if it came down to it, she would make her fall.
I only had one horse try to keep going with me. With one rein, leaned forward, turned her just a bit toward me and looked her in the eye and bumped her with that rein, she responded. I think she was just having a good time and forgot about me for just a moment. All the other horses I have ridden (and for that matter even handling from the ground), I teach them to circle. So they learn if something spooks them, they can move, but in a circle. When it comes to spooking, it is fight or flight and sometimes if you try to make them "whoa" you might get that fight aimed at you. Normally after a couple circles, they will settle back down. There is no fighting with them and they begin to look to you for the answers.
And NO, I do not claim to be a trainer. Just drawing from my own personal experiances. From working with the guy that raised Apps, raising my own, and my Ex Father-in-law was and is still a horsetrader so we always got to be the guinea pigs and take the horses out riding and see how they handled and rode. I look back at that now and realize I'm probably lucky to be alive cause some of those horses were barely green broke