I just found one for cheap, and thought I’d give it a shot for my very soft-mouthed, will go behind-the-vertical when stressed horse.
I love it! He loves it. He’s been consistently reaching out and down to the bit, taking a much steadier contact that I can really work with. It’s just a plain loose ring with a lozenge in the middle. Aside from the neat difference in the contact, when I bridle him I just sort of put it near his head and he goes right for the bit, and sucks on it for a while while I’m taking the bridle off.
I figured I’d let others know that at least for me, it works for this kind of horse.