Yellow wire is Enable. If Enable is High, the H-bridge works and the motor will spin (or not) depending on the D1 and D1 signals. If Enable is Low, the H-bridge is off and the motor is like not connected, that means it can spin freely (or coast).
You want the Yellow wire to be connected to 5V (High) and PWM one of the Direction pins (D1 or D2). The other direction pin will actually set the direction of motor spinning.
Also, the Red wire is to be connected directly to the battery, not to the regulated 5V, but be careful not to excede 12V. Regular servo motors are powered up to 7.2V, so that should be fine.