A timing light is a very bright "strobe-style" light that flashes each time a cylinder fires. It is normally connected to #1 spark plug wire, and is powered from the battery (+12V and ground). You start the engine and point the strobe at the timing marks. Since you can't see the flywheel marks as they pass a stationary point used as a timing reference because of the speed of the flywheel, the stobe makes the marks appear to stand still. You can therefore adjust timing as required.
