With the ignition tester connected between the high-tension lead and an engine ground, spin the flywheel. If a spark jumps the gap between the electrodes, you may assume that the ignition system is functioning satisfactorily.
If an engine is misfiring, connect the ignition tester between the high-tension lead and spark plug. With the engine is running, an ignition fault will be seen as a ‘missing’ spark jumping across the electrodes.