The microcontroller has to be connected to the motor controller (OSMC) so it tells the OSMC how much power to send to the motor, and in which direction - forwards or reverse. There is a safety signal that tells the OSMC whether to lie dormant or go "live", there are two other pairs of wires controlling the type of pulsed signal to send to the motor. The microcontroller sends a pulse width modulated signal (on and off very fast) at 5V to the OSMC. The OSMC then sends a high power equivalent of this signal to the motor, so controlling its speed. This so called PWM signal is such that the more of the time the +5V is "on" relative to it being "off" i.e. 0V, the more power is sent to the motor. This "on/off" PWM signal cycles very fast e.g. 20KHz which incidentally is why the motor makes a high pitched whine. Here you can see the ribbon cable from the microcontroller running to the OSMC.