Here is a very informative write up on the ESC's.  Give your self about  30 min to read it and look at the graphs.  This helped me to even beter  understand what is goin on with in our aircrafts.  Simply mind boggling  and amazing.

http://modelaviation.com/inside-esc

ProQuad,
             Excellent informative write up , The author explains every thing to a T --- Only one acromion he
             did not explain  --- I would like to add for a further more better under standing --- he explained
             what MOSFETs, or FETs for short -- Metal Oxide Semiconductor Field Effect Transistors -- But
             not PWM, It means (Pulse With Modulation) Mainly used with DC voltage circuits The faster the
             the Pulse is to the motor the faster it will rotate. EXCELLENT -- it is a 10+ read, All should read it.
FlySafe-FlyFree   
            

I found the PWM/PWM-rate discussion could be explained better. The rate of pulses determines the speed of the prop.  PWM (pulse-WIDTH-modulation) is the time width (ON time)  of each pulse and is a method of delivering more power (or less) to the motor (at the same rotation rate).  The main purpose of PWM seems to be to provide the minimum power necessary to keep the motor rotating at the rate set by  the pulse frequency.  I can be likened to an automobile traveling at a constant speed needing a bit more gas to keep it going up a hill at the same speed.