Originally Posted by RONALDO_UK
...it certainly doesn't control the voltage...
Actually that's pretty much what an ESC does.. it controls voltage on it's output side (voltage on the input remains at battery voltage).
It does this by by turning the power on and off very fast, so if it's supplied with 12v and it outputs on-off pulses that is off 50% of the time (zero volts) and on 50% of the time (12v) then the average voltage output is 6v. Because the rate that the power is turned on and off is so high the motor responds exactly like it would if fed with smooth 6v.