Looks vaguely like something I saw Monty Roberts use. I have found a clip on youtube of the demo I saw - not very clear. Might be worth cruseing the web for info from him about how to use it and how it works if you are interested. Although you don't see much of it in this video I can tell you when the horse came in he really did buck!
I saw Monty Roberts use something like that on TV. Someone did end up tentatively riding the horse after LOADS of the join-up, fannying about, dummy on it etc... I'don't know how well it would work on a fresh horse, or if it would even make things worse. Interesting though.
I wonder what would happen if the horse stumbled or fell? It may even lead to the horse falling more easily if it cannot use its head to balance itself.
I guess it might have it's place . . . but I can't see why you'd want to prevent a horse from raising its back as this is kind of the point of schooling and I don't really believe you could work a horse properly in it.
My guess is someone experienced wouldn't use it and someone inexperienced to need it shouldn't be on the horse. The ONLY case I MIGHT see it being useful is a school-type horse which has learned to yank its head down with inexperienced riders and even then, there are other ways to deal with that.