The D-Ring snaffle is somewhat similar to the Eggbutt snaffle, in that the ends of the mouthpiece merge into a hinge, to which the ring attaches. This shape stops the corner of the mouth from being pinched between the mouthpiece and the rings.
The D-ring gets it's name from the shape of the bit rings.
D-Ring bits are popular for use on racehorses.
This particular bit has copper and steel rollers on the mouthpiece, which encourage the horse to salivate, making him softer and more responsive in the mouth.