A Ring Cheek piece introduces poll pressure as well creating leverage on the mouthpiece. The 4 Ring has 2 rings on the bottom to adjust the amount of poll pressure and leverage you want.
The Colin Miles is a great schooling bit with a 55mm 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.