One of the most useful training exercises for horses is to use rollbacks along a fence as a way to develop softness and flexibility. It also helps teach the horse to “listen” for cues.
You initially walk the horse parallel to a long fence, about 6-8 feet away. You’re going to turn right towards the fence. Pick out a spot ahead and when there softly say “whoa” and wait for a complete stop. When first teaching a horse this skill you might want to have him take a step backwards to cause a weight shift but as soon as you can just do the stop and let the horse do the weight shift on his own.
Use an indirect rein to the right, put a heel against the horse on the rear right side. I usually shift my weight slightly backwards and towards the right rear but some trainers stay centered and erect. The horse should stop, pivot around 180 degrees and start walking again. The second he begins to turn correctly release all pressure. You want him to learn to voluntarily turn, you don’t want to have to pull his head or bend his neck.
You’d repeat the same thing further down the fence but this time with opposite cues to turn him to the left. You just go back and forth like this until it feels natural. Then increase the speed to a trot. You can do it at a lope but I’d only do it a few times during a session, the fast stops at a lope are hard on a horse. Once things feel good do the same thing away from the fence.
The key is at the stop having the horse shift his weight to the rear. He can then pivot directly on a rear foot and throw himself around to face the other direction. Ideally, he should eventually rock back far enough so that he can perform the roll without bending his neck at all.
You can also teach horses to do rollbacks when you are on the ground. You can practice it in the round pen or in an open area with a long lead rope. Circle him, cue a stop, cue a “other direction and go”. There’s an exercise you can do in the open where you talk towards the horse with a long lead rope (or without if you’re really good!). You get him to half circle back and forth in front of you doing rollbacks to change direction. Some folks call it a “waterfall”. My quarter horse Summer Soda loves doing that.
It also greatly helps if slightly before you stop you turn your head and look where you want to go next. The horse can definitely feel that movement and it helps cue him. If you want to get trickier, after he learns what to do, don’t do any hand/feet movements or just very slight ones but do turn your head and then move your shoulders down and then up as you turn your upper body so you are replicating what you want his body to do. It feels great when you are both in sync and making the turns together.
Reining competitions use rollbacks after a stop and they are mostly judged on all of this being one continual movement (not a stop and then a turn) and of course making it look effortless.