Basic light using an orange neon discharge tube connected in series with the HT lead. It works, but needs to be used in low ambient light conditions with the timing marks clearly marked with white paint.
More sophisticated 12v style with a very bright white xenon discharge tube, easily viewed in daylight. Has 12v and earth connections for the electronics, and a clip-on pickup than can easily be moved from lead to lead while the engine is running. Versions available with an adjustable dial which is very useful for checking advance at various rpms, others with a digital tach/dwell/volts display. I dislike too many functions in one device, for a start it makes them much more expensive, but if any one of them fails either you have to scrap the tool and buy another, or buy an additional tool for the failed function, which you could have done in the first place.
Turn the knob until the flashing shows the timing marks at TDC, then the pointer on the knob indicates the amount of advance - simples.