A serpentine belt tool is a specialized flat bar wrench designed to release drive belt tensioners. Modern front-wheel-drive cars have engines mounted sideways, leaving very little space between the engine accessories and the vehicle frame. Standard ratchets and sockets are too thick to fit in this gap. The serpentine belt tool is extremely thin, usually under a quarter-inch thick, allowing it to slide down to the tensioner pulley bolt easily.
The tool uses a long handle, often over two feet long, to multiply your turning force against the stiff tensioner spring. It comes with interchangeable shallow sockets or crows-foot wrenches that attach to the end. You slip the socket onto the tensioner center bolt and pull the handle to compress the spring. This releases the tension, letting you slip the belt off with your other hand. Never place your fingers directly between the belt and pulley in case the tool slips.