It's a gadget called "Light Keeper PRO". It looks like a toy ray gun
(Google it for a picture and where to buy). All you do is plug in the
defective light string, remove one of the unlit bulbs, plug the empty
bulb socket into a socket on the device and pull a trigger. It forces
the shunt in the burned out bulb to operate allowing the remaining
bulbs to light. Replace the bulb you removed and look for the burned
out bulb that caused the problem. My guess is that it generates a
electrical pulse similar to a gas grill starter to somehow force a
defective shunt to operate.

It does. It has a piezo element.

It's quite an elaborate gizmo. The user manual is huge.


In case the problem is caused by a wiring defect of some sort, the
Light Keeper PRO also has a audio voltage detector for tracing the
electric field in the wiring. It also tests bulbs and fuses.

So far I've repaired 2 light strings. In both cases it was due to
faulty bulb shunts so I haven't had occasion to use any of the other
options. When those lights fail while in use outdoors it's a real pain
to manually troubleshoot.

I found mine at a Meijer store for $20. I suspect they can be had
cheaper if you look around.

I got one at a local hardware store for $10. I also spoke to the owner on
the phone. Nice guy.