Sprenger's Max Control ("Danger") Bit is popular because of the way it encourages the horse to soften.
When the horse is soft in the rein, the bit works like other double-jointed loose ring snaffle bits. But if the horse resists against the hand, the mouthpiece locks to act like a stronger straight bar bit until the horse relaxes and works properly again.
Stainless steel 16mm mouthpiece and 70mm rings