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.
The Colin Miles is a great schooling bit with a 55 mm center piece creating greater tongue pressure and reducing bar pressure. The rings give the horse something to play with and help keep the horses attention on the bit.
This particular joint exerts more pressure than the Control Plate or Elliptical.