When you pull on the reins, mouthpiece goes up, poll preassure is applied. I shouldn't think they're massively severe, slightly milder than a duch gag I should think.
yup we have a little riding school pony in one who was forever shoving his head about and charging off the dumping the kids in a corner-it's worked a treat!
Yes they work and bring the head down, they are actually a nice bit for someone who just needs a little more than a snaffle, but not quite as much as a 3/4 ring gag.
If you are hunting for them online they are also called "beval" bits.