The blue wire is for power of the motor inside the servo. It can connect to voltage higher than 6V ( from the receiver). It can be 7.2V, 8.4V or 9.6V to increase the speed of the servo. This wire is use to separate the power of the servo motor to the control signal. You should not connect to the blue wire of the original servo cord as it will cause short circuit. You can only connect to the red wire of the sevo cord but it will slow down the speed of the servo as the input voltage is 6V only. Masami use 2 additional battery ( small size ) for the steering servo ( the first KO FET servo ) to speed up the servo when he win his first world title.