It's quite clever if you think about it:
A lens takes all the light outside then shrinks it down to fit on the film/sensor in the camera. So if your reverse that it takes a small area if light and increases it. But, the reversed side of a lens is bigger than film/sensor... So you attach that to your normal lens.
The light gets increased like a magnifying glass by the reverse lens, then scaled down to fit the sensor in the camera by the normal lens(which is mounted normally). It allows you to take REALLY close up images without buying an expensive specific macro lens.
My camera has a minimum focusing distance of roughly 8cm i think. But using an old 50mm lens screwed on to the front in reverse I was able to focus at less than 1cm. A 1cm area is filling the whole frame and so i can crop to get tiny details. It's how some people take photos of the eyes of insects and stuff.
Here's some test shots I did. (note I'm inside using shit lighting so the shots are quite noisy at ISO3200, outdoors in daylight they would be even clearer):
How exactly do you go about screwing a lens onto another lens, backwards? The front of the lens isn't threaded, right?
