A strobe light used for setting the ignition timing on an engine, a typical timing light has three connections; one positive power connection, an earth and a trigger input.
When setting the dynamic timing on a car, the engine must be running at a set rpm, and the first spark plug must fire when the crank is at a specific angle before or after top dead center. (Static timing is set with the engine turned off).
There is usually a mark on a moving part of the engine, such as the fan-belt pulley. There is a fixed scale on the stationary part of the engine next to this mark. The trigger input of the light plugs inline with spark plug 1, and so flashes every time the plug fires. This seemingly freezes the movement of the engine, and the marked part can be compared against the fixed scale.
The timing light can be dangerous, since it seems to freeze the engine. It is easy to forget that it is actually spinning at high speed. All loose clothing and hair must be secured, and you must make every effort to remember which parts are moving and avoid touching them.
Most timing lights are shaped like a gun, and have a finger trigger to operate the strobe. This ensures that the engine appears frozen only when the scale is actually being read.