Julius von Bismark has discovered the true power of a portable flash unit with an optical slave setting.
Basically, an optical slave is a second camera flash that fires whenever your primary flash does. The sensor sees flash-intensity light, and it fires. Normally, this means you can have it sitting on a stand a couple of feet to the left and get a nice side highlight in your portrait.
Herr von Bismark has realized that he can also use this to project light onto other people's photos. As you can see in the video, he's stuck a flash unit into the back of an old film camera. The camera has already-exposed slide film loaded; and I'd imagine he has the shutter open permanently. Basically, he's turned the camera into a slide projector that only projects for a fraction of a second, and only when someone else takes a picture using a flash.
Here's an interview where Julius briefly discusses his reason for patenting this invention: basically, if he doesn't patent it, make it known, and use it for creative purposes, someone else will inevitably use it for evil. Perhaps they will anyway. Even with a patent, it would be difficult to stop advertisers and the owners of photograph-worthy landmarks using this to their advantage. (Imagine "buy this album on iTunes" on all your concert photos.)