The technique is used to take very short distance/true macro (1:1 or less) shots with available lenses. The disavantage of this technique is the VERY shallow DoF (usually a couple of mm) so is really difficult to have things in focus.
I don't know the technical explanation, but this is how it works:
You need a SLR lens, preferably a 50mm prime one, to avoid chomatic aberrations and softness. Any lens from any camera will do the trick. I've used a 30years old thread mounted 55mm Pentax lens, from my Pentax Spotmatic camera. Also you need a camera with its lens. Can be any kind of camera, but you'll need a threaded lens in it to screw the adapter ring if one is used (recomended).
You must attach the SLR lens backwards (with the front element facing your camera front element) using a ring specially designed for that function, or, as in my case, just slide one lens inside the other. If you do the last option, be careful the lens elements aren't in contact with anything or you will ruin your optics. In my case the diameter 49 SuperTakumar fits almost like a glove inside the diameter 52 Canon, and there is no optics near the meeting point in the canon one, so it was a safe operation.
The camera lens must be at least a short tele (80mm equivalent or more) to avoid vignetting. My Canon 50mm lens wasn't long enough, and a lot of vignetting was present. I've cropped it in the final picture.