Shumway.
On a light aircraft, the 28V power to the starter motor is supplied through a solenoid which is a heavy duty relay with contacts that can handle the start current. The coil of the solenoid to close the contacts is an inductive load which is energised by the start switch. When the start switch is opened, the inductor will resist the drop in current and the stored magnetic field of the coil will create a back e.m.f. This emf, as pointed out in a previous post, can be extremely large and many times the battery voltage. If this high voltage creates an arc across the start switch it will be connected to the battery bus bar momentarily and any electrical equipment on that bus bar. Radio equipment switched on is particularly susceptible to any voltage spike generated in this way.
A more technical description of this can be found via this link
Flyback diode - Wikipedia, the free encyclopedia