With this permanently mounted highly conductive layer, any flux movement creates large currents (Eddy Currents) that can now short themselves out.  Rather, the magnetic flux lines are electrically shorted to their original location and cannot move now.  Hence the devices used for this are called Shorted Turns or Faraday Rings. The benefit is a great reduction of distortion and overall inductance of the driver, as well as linear inductance throughout the stroke.

Another benefit of this copper ring is that not only is it highly electrically conductive, it is also highly thermally conductive. While steel is able to absorb high amounts of heat quite well, it does not do so very quickly.  The VC needs to quickly dissipate the heat that is generated as power is applied. If this is not done, the VC temperature will rise leading to a rise in the resistance of the wire. This leads to power compression. As this happens we need to put more power into the VC to keep the output the same.  Hence we start this chain reaction that we will never win against. The VC at some point absorbs double (3dB) and even quadruple (6dB) the input power for no gain in output Spl.

So our "perfect" motor has to then have a large piece of copper designed in such as way that no matter where the VC is located, the copper is right there to absorb the heat.  The copper serves as double duty this way by also keeping the flux field from moving no matter where the VC is.