Imagine a circular disc with a dot painted on it, towards the edge. If you rotate the disc, say 50 time per second, the dot will appear as a circular ring. This is because we see it at different positions during the rotation. Each rotation takes 20 milli seconds. Suppose we illuminate it using a pulsed light source that glows for a millisecond n every 20 mS. The dot will be illuminated only when it is at a particular angle. To our eyes it will appear stationary.
When the frequencies are very close, the motor will appear to rotate at a frequency that is the difference in the actual frequency of rotation and the frequency of illuminating light pulse.