IMPORTANT: This timing light is for use on 12 volt ignition systems only.The exact moment when the spark plug ignites the airfuel mixture to create an explosion and deiver the power to the engine is called " timing "Timing is given in degrees or inear measurement ( e.g. milimeters ) relative to the top dead center (TDC ), of the movement ofthe piston. BTDC: Before top dead center ( timing advanced ). ATDC: After top dead center ( timing retarded ).