Pulse Width Modulation aka PWM

This is the method of generating the appearance of dimming, or variable voltage (if you average) by outputting a series of on and off pulses, the different values being generated by altering the ratio of on to off time.

Such techniques allow us to get much smoother and more linear results from devices which would not have reacted so well to lower voltages, or where we simply dont have the ability to output a lower voltage.

One of the classic uses to adjust the aparent brightness of an LED, the brightness of an LED varies with the curent through it, not the voltage, but by flashing it on and off more quickly than the eye can see, we can achieve a large range of brightness values as our eyes simply see an average of all the on and off pulses.

Motors too can benefit, as although they can change speed through simple voltage changes, you can get much finer control, and much better low speed performance by using full power pulses with an appropriate spacing between pulses.