A simple way to think about inductive effects is to remember that the current in an inductor acts similar to the inertia of a mass, it wants to keep flowing, even after the voltage (force) across it it removed. It will keep flowing until all the inductive energy (1/2 LI²) is dissipated, which can generate a very high voltage if there’s not a path for that current.
A diode is connected in reverse so that when a switch / relay contact is opened the current that will flow due to the energy stored in the inductive load will flow through the diode and be dissipated. The fly wheel diode provides a means of applying a brake to the fly wheel.
When the supply voltage is connected the diode is reversed biased and is effectively out of circuit. When the switch opens fly wheel current produces a back EMF in the opposite polarity and so the diode will conduct. The diode does a very good job of suppressing the back EMF and clamps the voltage to around a one volt or so.
This is good for say small solenoids, etc. However for big inductive devices (e.g. a large door hold magnet) the power dissipated by the diode is low because the forward volt drop is less than a volt. This means the rate of energy removal from the electromagnet can be low because Energy ≈ Power x Time. 0.6V conduction of the diode results in the energy taking longer to be dissipated – the electmagnet of a large door hold may take a second or two to release for instance. For these applications a MOV is often a better choice.
MOV – Metal Oxide Varistor
A MOV is a voltage dependant resistor (VDR) and has a rated voltage. Below this voltage it will have a very high resistance. If you select a MOV with a rated voltage slightly higher than the normal supply voltage it can be safely connected across the coil of an inductive load magnet and just like the diode has no affect while the supply is connected. When back EMF is present the voltage rises to the trip voltage of the MOV at which point its resistance drops dramatically and the energy is quickly dissipated. Because the voltage is higher than a diode the energy is dissipated much faster.