Most of the time your horse (hopefully) stops on cue. You increase the contact on your reins and simultaneously ask your horse to slow by stopping his forward motion with resistance from your seat.
On rare occasions, you may need more stopping power. It could be that your horse has bolted or started bucking, or you could be out galloping with a group of friends and find you need just a bit more than normal to get your horse under control. The one rein stop is a tool that you should (and your horse) should know for those “just in case” times as it helps break the horse’s longitudinal resistance (by moving over his hindquarters he cannot keep running straight). But it’s not something to try for the first time when you need it; it’s something you need to teach your horse in advance. This is important: if you try it for the very first time on a horse that’s running in a blind panic, you may well flip it over onto it’s side. If it’s a technique that’s been trained, simply bending the horse will encourage it to slow down.
While this is a technique that is commonly used by Western riders, it is more frequently being taught to English riders as well. Learn how to train the one-rein stop.
Step 1: Before you try this exercise mounted, it’s best to teach your horse how to move its hindquarters over and bend its neck from the ground. Stand beside your horse while he’s tacked up. Bend his head in toward you. When the horse has accepted the bend and is standing still, then place your hand just behind the girth and ask your horse to move its hind end away from you, keeping his front legs more or less in the same place. Rinse and repeat on the other side.
Step 2: Once your horse understands the concept, then try it mounted. At a halt, ask your horse to bend to the left while releasing your contact on the right rein. Don’t use your leg and don’t bring your hand behind your body. A simple approach is to place your hand on your thigh. Wait until your horse stops moving and releases into the bend. Next, gently apply your left leg and ask your horse to move it’s haunches away from the pressure. Make sure you repeat on the other side. In addition to teaching your horse the one-rein stop, bending each direction helps keep his neck supple.
Step 3: Try the one rein stop a the walk. Apply the same principles. Then move to a trot and finally at a canter. Remember when you try this at a canter to take note of the lead that you’re on — you want to turn in the direction of the lead. Remember, the idea isn’t to spin your horse off his feet, rather it’s to use the exercise to get your horse to stop because you’ve bumped his hindquarters over and caused him to slow to regain his balance. Don’t haul your horse’s head around, especially at higher speeds; rather you should give and take on the rein you are using to ask for the bend.
Step 4: Practice this periodically. This should be part of your regular training so that if you ever need it, your horse knows how to respond.
Here are two videos that give examples of how to train the one-rein stop: