The Koni Special Active Shock FSD 97-03 BMW E39 5-Series Rear is an adjustable, performance shock absorber designed to improve the handling and ride quality of your BMW E39 5-Series. It features a unique Frequency Selective Damping (FSD) technology that allows you to adjust the damping force for each wheel independently, allowing you to tailor the suspension to your driving style and road conditions. The shock absorber also features a progressive damping force, which reacts to the road surface to provide a comfortable ride while still providing maximum control. This shock absorber is the perfect choice for those looking to upgrade their suspension system and take their BMW E39 5-Series to the next level.