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.
A conventional Snaffle joint has a nut cracker action, which means it will bear pressure on the horse's bars and the tongue. The Barrel joint limits the nut cracker action and spreads the usual pressure over a larger area. When the rein is used this mouthpiece wraps across the tongue creating tongue pressure and taking pressure off the bars due to the lock up action. Please note appearances may be deceiving, this bit joint is stronger than the Elliptical as pressure is more concentrated on the tongue as this joint is smaller.