kf2qd:
Called a FLux Ring. Improves the magnetic field around the motor to make the motor have more torque. Also improves efficiency. Some motor designs need it more than others. For instance - I have built some brushless motors for model airplanes. Without the flux ring they could run faster, but with little torque. With the flux ring they have much more torque and are much more usable for spinning a propeller.
And if add to the flux ring a flux capacitor of the proper size your motor will take you back to the future. 