The Colin Miles Blue Sweet Iron mouthpiece from Bombers Bits has a 55mm centre piece with rings. This creates tongue pressure if the horse becomes strong and also gives the horse something to play with, which encourages a soft contact and helps to keep the horse's attention on the bit.
The Petros mouthpiece rotates, which can help to prevent the horse from leaning or trying to take hold of the bit.
The blue sweet iron mouthpiece encourages salivation, softening and acceptance of the bit because it oxidises easily, creating a warm, sweet taste in the horse's mouth. The double joint means the mouthpiece follows the contours of the horse's mouth, which reduces the 'nutcracker' action associated with single jointed bits, and resultant bar and palate pressure.
The loose rings make the bit mobile within the horse's mouth, further encouraging playing with the mouthpiece and relaxation. They also allow an immediate release of tongue and bar pressure when the rein aids are relaxed, thus rewarding a correct response to the aids.
Please note that the blue will fade with use.