OK, I will make this the simplest explanation there is. lol
Have you ever turned off the faucet really fast after running the water full blast? When you do this the pipes bang. Same thing. Its the water abruptly coming to a stop.
This the just what happens with the voltage when you turn off during each cycle of the pwm. The voltage spikes and smash's into the closed gate of the mosfet. The diode is basically a pressure release that allows the voltage to flow back into the battery and not pound the mosfet to death.
-Adam
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