The Rear Extender Upgrade is a footlong cable extension that attaches to the top of the shock's damping adjustment knob in certain applications. This allows you to route the adjustments to an easier-to-reach location inside your trunk or hatch area. Typically requires drilling a 1/4 inch hole through any interior body panels. The rule of thumb is that they are unnecessary if you can see or reach the top of your factory rear shocks/struts. Some applications use special adjusters to mitigate the need for rear extenders.