The Williams cheekpiece allows for multiple rein positions. If connected to the main ring the bit acts as a loose ring, but when connected to the bottom ring it promotes pole flexion by creating more positive poll pressure. The Williams cheekpiece helps to prevent drifting.
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.
Recommend this style to be supplemented with Bit Guards and allowing 10mm extra to sizing for the cheekpiece if your horse has fleshy lips.