It is commonly called an elevator or a dutch gag. It is a leverage bit, frequently used without a curb strap. Rides have the option of how much leverage they want with the multiple rings. The big one causes the bit to basically be a snaffle while lowering the reins to the bottom rein adds more leverage and in some cases more control.
That specific one looks like a Herm Sprenger elevator bit.
Mickie So Fine - You are right! The lighting in the picture makes it look like the big ring is open from the ring the cheek piece is attached to to the actual mouth piece. That's what got me so confused!