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.
This is my best guess, but I'm completely unsure: When the NPN BJT is in forward active mode (i.e when the LED is turned on), current will flow from the +6V rail to the -6V rail. There will be a potential difference across the 1K resistor and therefore a potential difference between the gate and source of the n channel FET. The FET is now in conducting mode, which will then reverse bias the diode and turn on the motor.
When the NPN BJT is in cut-off (i.e when the LED is turned off), the FET will act like an open circuit, so no power goes through the motor. Under no circumstance is the diode ever forward biased.
Is this correct?
And what is the capacitor and Rx for?