Moving Iron question

As I understand it, the motion of a magnet relative to a fixed coil will induce a varying current in that coil. Similarly the motion of a coil relative to a fixed magnet will induce a varying current in the coil. How does a Moving Iron Cart like the Grado or Soundsmith induce a current when the moving element is neither a coil nor a magnet. Any help here?

Thanks in advance,

Marty