IF the ESC has 後波 it needs mosfets in bridge configuration plus it need n-channel and P channel mosfets to control the motor, i.e. the current will go through two mosfet plus the motor. 無後波 ESC the current only go through one n channel mosfet plus the motor. The Rdson (or =resistance, you may see it on the box of the ESC) of 有後波電變 is much higher (higher resistance lose more power) than 無後波電變.