This electrical device is used to show your engines timing in real time, to make adjustments.
How to use a timing gun:
Timing guns will have a power lead and a lead that will be connected either directly to the #1 spark plug or an inductive clamp that will wrap around the #1 spark plug wire.
Timing gun as seen in Basic Engine Building:
Pointing the light at the timing label on the harmonic balancer while the motor is running, will tell you how advanced or retarded the ignition timing is. Here we are using the timing gun to set our initial timing for our motor.