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 curb gag has a sliding rope action on the cheekpieces, which applies pressure to the horse's curb groove via a leather pad, rather than the strong poll pressure applied with a conventional gag bit. There is still a small amount of poll pressure, as the upper ring acts in a similar way to a hanging cheek snaffle.
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.
This bit is designed to be used with two reins, so the curb pressure only comes into play when it is really needed.
Please note that the blue will fade with use.
Please note that the curb gag is only legal for the jumping phases of eventing under FEI rules if two reins are used (with one attached to the main ring of the bit).