The Dee Ring is very similar in action to the Eggbutt, it will prevent pinching of the lips and gives a slightly more solid feel against the side of the face. It also prevents the bit being pulled through the mouth. The release is slower and some slight poll pressure is introduced.
A horse shaking its head when bit pressure is applied or its tongue trying to escape the bit are classic signs the horse has a sensitive tongue. With this in mind the McHardy was created, it has a medium port creating space for the tongue relieving pressure. The Buster Roller in the middle increases bar pressure and as a result lifting the horse's head and giving more control.