This basically consists of a monitor and piece of polariser. However the monitor has had the outer piece of polariser scraped off. This means that all the light that gets through the rear polariser can always get through. But if you look through a polariser you can see the picture again. Rotating the polariser by 90 degrees will of course let through all the light which was previously blocked, so inverting the image.
This shows how a piece of physics can be used in something we use every day.