I'm a huge fan of one rein stops! I teach them to every horse I get on. I use them pretty regularly and not just for emergencies. For example, a horse that jumps, drifts in, and cuts the corner? One rein stop to the outside at the end of the line teaches them to keep straight. Or if they drop their shoulder around the corner? One rein stop to the inside gets them off their inside shoulder (or one rein neck bend is actually better for this). Or if I get on a horse whose owner hangs on their mouth and teaches them to pull, I don't even regular stop the first couple of rides, I one rein stop until they realize that I'm not going to hang on them and they need to be soft.
I did have to use it on a paint horse I'm working with. There's a 6 ft privacy fence a couple of feet from our arena where someone was raking leaves (or something loud in the leaves) on the other side. You could barely see glimpses of him through the fence but my horse was convinced it was a monster. He leaped to the side and bolted so I one rein stopped, but as he turned he faced it again and tried to bolt again. So we were kind of running and leaping around a bit. :roll: But then he eventually stopped like was supposed to!