Final answer:
When backing up straight, you place your left hand on the top of the steering wheel and guide the car in the desired direction by moving the hand accordingly, utilizing the function of your forearm muscles.
Step-by-step explanation:
When backing straight, put your left hand at the 12 o'clock position and move the steering wheel in the direction you want the back of the car to go.
The superficial posterior compartment of the forearm which is involved in movements such as straightening the wrist away from the body and tilting the hand to the side away from the body, demonstrates the biomechanics behind steering a car.
In the context of driving, this means that when you want the rear of the car to go to the right, you would move your hand in a clockwise direction, and to move the rear left, you would move your hand in a counter-clockwise direction.