
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.

The Mikmar D-Ring Snaffle Bit
incorporates the classic D-Ring style bit with our Mikmar double-jointed slanted low port mouthpiece. It is to be used without a curb strap and provides good control with excellent vertical and lateral flexion.
The unique design of the new Mikmar Snaffle bit unites classic looks with superior results. The traditional style cheek pieces are made from the highest quality stainless steel available and are combined with our Mikmar double-jointed slanted mouthpieces. The lightweight mouthpieces are flat and wide and incorporate a copper roller and a low port. The double-jointed mouthpiece is comfortably and properly positioned in the horse''s mouth for effective results. The Mikmar Snaffle bits offer good control and aid in lateral and vertical flexion and proper carriage. These bits are well-suited for the show ring as well as pleasure riding and present the desired classic snaffle bit looks with optimal results.
http://www.mikmar.com/bit-pages/d-ring-snaffle.html