[Beaker]

Now I'm using a mullen mouth with a moveable piece on it....sorry I don't know the real names and what you call the little piece that rotates in the middle of the bit
Sounds like a roller to me. It's supposed to keep the horse "entertained" or more willing to accept the bit. I had an eggbutt snaffle with keys at the joint. The horse liked it, but it irritated the heck out of me.

[HairOfTheDog]

The roller also serves the function of making it harder for the horse to get his tongue over the bit, if he has learned to do that. I would rather they were just quiet in the mouth than playing with something, too.