I use this one on the Arab I ride
.com/Coronet-Copper-Roller-Eggbutt-Snaffle/dp/B0011UDLWMHe was in a single-joint copper snaffle that made him raise his head high to get away from the nutcracker-effect (he has a shallow mouth). I put him in a standard french-link, and he fiddled with the flat center-joint...fiddle fiddle fiddle. It seemed like he "needed" something to play with, so I got him the one in the link, thinking the roller would make him happy, and it did. Oddly, he doesn't play with it at all. I think he just like the feel of the round center-piece on his tongue. Different horses just like different things and we just don't know why.