Gag bits are a type of elevator bit that act both through the mouthpiece but also by applying pressure on the poll. Special cheek pieces are used for the bridle that run through small rings in the bit ring and attach to the rein. The effect you then get when you pull on the rein is that the gag is raised upward in the mouth and also creates pressure on the poll.Read more
Curb gag is a similar bit but acts instead by applying pressure on the jaw. Both of these are sharp bits and not for beginners, but if used correctly can be helpful, usually for horses that become heavy on the forehand or a bit "stretchy". A good idea might be to ride with double reins to better control the elevating function and manage it during the session, even the delta can work well for some.