how do bodyfat monitors work?

how do the bodyfat monoitors that you put your hands on work? how does it track your bodyfat? i hear they are very accurate


example:

http://www.mypharmacy.co.uk/health_produ...fat_monitor.htm

The body fat percentage is measured based on the electric resistance and the personal data such as height, weight, age and gender. The measured result are closely correlated with "underwater weighing method" and the DEXA (Dual Energy X-ray Absorbtiometry) method, which are said to be the standard measurements methods for measuring the body fat percentage

The home market electrical impedence kits are notoriously innacurate. Even professional bodystat units have to be used with strict protocols to ensure consistent result measurements. Caffeine, food and alcohol all have to be avoided for a minimum of 4hrs, and preferably 8-12hrs prior to the test. The best results are achieved with systems that run the current from wrist to ankle, and you must be lying flat on your back and be settled for a couple of minutes prior to running the machine.

A battery powered hand grip only machine, or a set of bodyfat bathroom scales really dont give you any results worth looking at.