OK:
Using Doppler allows the frequency of a signal to decrease when the distance between the beacon and aircraft increases, and vice versa. The Doppler shift makes the transmitter look as if it is advancing and retreating 30 times a second. That is, the aircraft sees a varying frequency rather than varying power.
As mountains don't move, they don't affect the reception of the signals and the VOR doesn't suffer from propagation errors. Do you need an explanation of Doppler?
phil