Blinkers are used for a number of different reasons, and the size and shapes of the cups make a big difference on how they work, and what they are good for. The variations in cup size and shape are very individual, how much you want the horse to see, and what you want to block for that horse.
They are often used on young horses, during breaking, if you happen to have a horse who is terrified of the sight of a rider on his back. Obviously, the horse is not terrified of humans on the ground, but that innate fear of “something on my back, like a predator” can just suddenly become a problem with a horse who has been well started and handled with his ground work. If he’s terrified of the sight of a rider on his back, blinkers block that sight and that terror for him, so that he can be ridden, and learn about everything other than the “sight” of the rider without feeling that innate terror. So he gets “broke to ride” without fear, learns “relaxation”, learns about leg cues, gets rewarded for good behaviour, all by putting off facing what he is innately afraid of. When he’s “broke” in every other way, there is more chance that he will not be as scared of the “sight” of the rider on his back, when it is re-introduced at a later time. Or, some horses will not ever be entirely comfortable with this, and need to wear blinkers for best concentration and racing performance indefinatley. When buying a mature horse off the track, it’s a good idea to find out if he has EVER been ridden without blinkers (if he’s wearing them currently), because he may not have been. And there may still be an issue with seeing a rider on his back. I had one who I NEVER rode without full cup blinkers in the morning, at the track or at the farm, due to his wierdness about seeing me on his back. As a baby, if my hands touched his neck, he would also freak. If a jock wanted speed in a race, I said, “just touch his neck with your hands”. If he knows you are up there, you will get full speed.
The other uses of blinkers are many, and again, the shape of the blinker cup is key, full cup, 3/4 cup, french cup, cheaters, holes cut to let him peek at a horse who might be coming up on him, or no holes. I had another who was a social butterfly. He would be in front, and WAIT for a horse to come to him, so that he could run WITH that other horse, in competition. It was no fun if he was out front alone. Unfortunately, sometimes the horse he was waiting for got past him, and we had “seconditis” for a while. French cup blinkers cured this problem, he didn’t see anyone coming to wait for them, kept him on the job.
The point here is, there are a selection of reasons why a horse may improve his performance with blinkers, and a number of different types of blinkers to effect many different issues that a horse may have, depending on that horse’s personality and issues. If a horse needs blinkers, and you put them on, many horses will heave a sigh of relief, and relax, simply because they no longer see “what scares them”. And for horses, often if they can’t see what scares them, that thing no longer exists, and a problem is solved for a rider or trainer.