Well, let's say you want to make a dark thunder. We'll use Zap1.blp for the example, but using another lightning skin might work betterly.
Here we got the skin, now we apply it to a plane with two faces. Face A and Face B, the unwrap will probably be like this:
We separate both faces in the unwrap, and put B where A was, and A where B was. And the plane will then look like this:
Which is an interesting texture to be used for modulate materials. It might work better with textures that have a contined lightning not like this one.