These things work on 'bio-impedence' , there's lots of variations around, including weighing scales which take the reading throught he soles of your feet, sticky pads which you place on either end of your body etc.
They measure the resistance and capacitance (impedence) of the body, and as the above post suggests, try to work out how much current etc is getting through,a nd do the calculations from there.
They fall flat on their face though, as the variables are just too great.
You are supposed to use them 'x' amount of time after/before eating/drinking, no alcohol blah blah blah, which effects the hydration of the body. Also, if you have particularly high resistance skin (resistance varies greatly between individuals) it will throw the readings out.
A few years back I tested with callipers and a bio-impedence machine.
The callipers came out around 12% If I remember correctly, and the bio-impedence registerd something like 19%. I was pretty lean at the time, and the person carrying the test out witht he bio-impedence machine, who was fairly experienced just looked at it and said 'I think we'll just ignore that!'
