The Kimberley Dee cheekpiece prevents pinching of the lips and gives a solid feel against the side of the face. It also prevents the bit being pulled through the mouth. The release is slower than a Loose Ring and also introduces some poll pressure.
The Colin Miles is a great schooling bit with a 55mm centre 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.