Matt is correct on this. The ESC is a square wave generator. It produces 3 seperate square waves (one for each wire to the motor.) the speed of the motor has nothing to do with voltage or amps, but instead the timing of the current fed into it. By increasing and decreasing the wave length of the square wave on the 3 phases the ESC causes the motor to spin faster and slower. Amp draw is a direct result of the work load caused at the faster speed.