I think as long as your not hitting a horse in the face or neck with it, its fine. I used an over and under with my retired barrel horse and I'd only hit him in the butt. With my current horse I use a crop but I don't hit him instead I hit my boot with it, it makes a loud noise and encourages him to go faster. But I usually use nothing and just kick with an occasional slap on the neck, not even a hard one. But my horse is a running machine I think over unders are good for the lazy horses.