How LightKeeper Pro Works In a normal strand of holiday lights, electricity flows from one end to the other, from light to light. If one light dies, something called a shunt activates, allowing the electricity to skip that dead light and continue to flow to the rest. The problem is, those shunts are fragile and often malfunction. That means the electricity can't flow, making you think all of your dark bulbs are "dead". That's where the LightKeeper Pro comes in. How To Fix Your Lights With your problem light strand plugged in, remove an unlit bulb and plug the strand's socket into the LightKeeper Pro. Now pull the trigger up to 20 times (usually it just takes 5-10) until the "dead" lights come back on! It's that simple! With a single light out, just use LightKeeper Pro to quickly extract the bulb and replace it with a new one. What LightKeeper Pro is doing, is forcing that problem shunt to activate. Thereby letting the electricity flow again like it should. But that's not all the LightKeeper Pro can do. Just look at these other amazing features! Features Use the voltage detector to find other causes of light failure. Like twisted or cut wires. Entire strand of lights out? Connect the AC plug to the LightKeeper Pro and fix multiple shunts Built-in Bulb Tester - Tests repaired and existing bulbs to make sure they work properly before climbing that ladder! Secret compartment in the handle stores spare bulbs and button batteries Three (3) 1.5 Volt button cell batteries included NOTE - This device WILL NOT fix LED-based light sets, larger lamped C7 or C9 sets, tube/rope lights and series light sets that have control boxes for motion or music.