I was told you need to have PCM if you fly any of the gassers because, of all the vibrations etc that can cause failure.
If you have electrical noise anywhere in the aircraft, PCM will not eliminate it for you.
PCM works on a square wave signal with a modulated carrier imbedded in that signal.
Basically it sends a signal to the receiver which it then cross checks and compares to the second carrier. If they match, the signal goes through to the servos. If it does not match, the signal is rejected.
If the signal gets rejected for more than a pre set amount of time, the Fail safe (if programmed) will kick in.
It is kind of like putting a square peg in a square hole. It fits and goes through. Try putting a round peg in and it won't go through.
In helicopters I have found fail safe to be useless. It means crash over here, or crash over there. Either way the outcome is the same. You can get away with it in an airplane because low throttle does not mean an imminent crash.
Peter