A horse that bolts is scary indeed because it’s like being aboard a runaway train. Though your first impulse is to stop the horse by pulling back with both reins, that’s usually ineffective. A panicked, determined horse is simply too strong, especially since they tend to lock their jaw against the bit. A better way to regain control is to disrupt the bolter’s forward momentum by exerting pressure on just one rein instead of two. You may recognize this method as a pulley rein if you ride English or as a variation on the one-rein stop if you are a western rider. In either case, it’s valuable as a sort of emergency brake.
Since most people are right-handed, we’ll use the right hand as the dominant one here. The instant your horse bolts, get both hands on the reins and shorten them so you have solid, consistent contact with your horse’s mouth. Then set your left hand down on the crest of your horse’s neck, just above the withers. You can grab a handful of mane for security if you’d like, but make sure the palm and base of your hand are braced against the crest. Then step down in your stirrups and lean back; tipping forward weakens your position in the saddle. Now, in one strong effort, bend your elbow and lift up and back with your right hand. With your left hand firmly planted on the neck, the bolter can’t merely bend his neck and keep running. He’ll have to tip his nose to the right. He’ll also be momentarily unbalanced, which will force him to alter his gait and pace to readjust his weight. Continue to exert your rein pressure until he begins to turn. If he resists, relax only your right rein slightly and then bend your elbow and lift up and back once more. As he responds, and unlocks his vise-like hold on the bit, lift your left hand off his neck just enough as a release so you turn the horse to the right. Once he’s turning, you can continue bending him into a circle and then stop.
Of course, having a horse that is supple laterally is more likely to respond quickly to this tactic. Practice with your horse, asking him to bend through his neck and yield or “give” to pressure from one rein at a time. Then, if he ever does happen to spook and bolt, you’ll be able to reconnect with him in just a stride or two.