Also known as a rubber donut, the job of the flex disc coupling is to absorb driveline shunts and dampen any annoying vibrations. This particular flex disc fits to the one-piece propshaft that runs off the xDrive transfer case, but over time the rubber naturally begins to degrade, and is exacerbated by the forces sent through the driveline. This is a quality replacement part from Febi-Bilstein. Made in Germany.