It's called a Pan Hard Bar and someone correct me if I am wrong but it helps in turning. It stablizes the rear end of the car from moving side to side.
it helps to locate the axle. It provides a solid arch for the axle to pivot on, and control the side to side motion of the axle. It dramtically helps the feel of the car in the turns.
they are the best with a torque arm bar. A must for Road Race cars. They keep the rear from moving side to side and dramatically helps turning. They are expensive though.