Two metal bars that are put under the rear axle of a car, so that when the axle twists during acceleration, the bars (which point forward) hit the underside of the car, and stops the rotation of the rear axle.
The point of this is to reduce what's referred to as "wheel hop", which is caused by the axle twisting by the momentum created between the axle and the ground, until the tires loose grip, and unwinds the axle (then you're back to square one, twisting the axle until it the wheels slip).
Also known as "slapper bars".