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The pull-bar is just one device that racers use to reduce "shocking" the rear tires as the driver gets back into the throttle. The link is spring-loaded and allowed to extend against the pressure of the spring to absorb some of the force of initial acceleration. Other types of devices for this use are lift arms and even leaf springs. The leaf type of suspension has a limited amount of wrap-up from the torque of the engine and acts like a pull-bar to some extent.