Gives a Visual Check of Spark Energy for Coil On Plug or Standard Ignition Systems. Connect the tool between the spark plug and the plug wire. The short wire is double ended to offer a straight boot and a 90-degree boot on one tool. With the engine running, the tool flashes a mirror image of the ignition spark. The additional 7" extension plug wire is used on coil on plugs that are recessed and hard-to-reach.