The Drive terminals are connected to a PWM signal generated by an arduino micro controller. The purpose of this circuit is to isolate the arduino from the motors to avoid too much current being drawn. What I understand so far: The motor will only turn when the diode is reverse biased. When it is forward biased this will short circuit the motor and turn it off. The opto isolator (LED and NPN transistor) allows current to flow from the 6V rail toeards the -6V rail whenever the LED is turned on. The rest confuses me. Can somebody please walk me through step by step how this circuit works? Thanks!