When the switch reverses the motor it is connecting disconnecting( connecting, disconnecting ) (giving power) to the crossed over wires. As this is the opposite same( opposite, same ) way round to the forward motor this will reverse break( reverse, break ) the motor and make the vechicle go backwards. If you did not cross over the two wires and connected them normally, the vehicle will go forward backward( forward, backward ) as this is the same as the forward wires. If you have the crossed over wires and then give them power they will turn the motor backwards forward( backwards, forward ) making the pinion turn backwards making the wheels turn backwards making the vehicle to go backwards.