Ad doesn't say HOW it works on the horse; just says encourages correct neck carriage & use of back. You need exact information, before you buy it.
I hope that you understand that the horse is never to be forced into a frame, never to have his head "set", but that proper training is the gymnasticising of the horse from back to front, leaving the HORSE to move and place his head where it's most comfortable for him, in response to your seat, weight, & leg. The horse knows better than any human how to manage his head, for whatever the human is cueing him to do; does this not make sense?
The horse is on the forehand when his mouth is lower than his hip bone, on a horizontal plane. If this contraption encourages the horse to travel with his bit below his hip, then it's damaging to the horse. The horse will break down from such forcing.
When bad training and riding has caused a phobia in a horse, so that he's compulsively tossing his head or some other such action, I see use for a temporary aid for his head, like running martingale, which keeps rider from getting bashed in the face. When tactful riding shows the horse that he needn't be slinging his head anymore, then the temporary rig comes off.