So how does it work?

 So I FINALLY had my initial meeting with my new personal trainer (my first actually, and it took over a week for the gym to find me one and get me an appointment).  Anyway, after going through all the paper work about what I wanted to accomplish, etc, she took my height and weight, entered that and my age into some little machine, then had me hold it (it had hand sensors) straight out in front of me and it "measures my fat percentage".  Does anyone know how it works and how accurate it is?

The Bioelectrical impedance analysis (BIA) method is a more affordable but less accurate way to estimate body fat percentage. The general principle behind BIA: two conductors are attached to a person's body and a small electrical current is sent through the body. The resistance between the conductors will provide a measure of body fat, since the resistance to electricity varies between adipose, muscular and skeletal tissue. Fat-free mass (muscles) is a good conductor as it contains a large amount of water (approximately 73%) and electrolytes, while fat is anhydrous and a poor conductor of electrical current. Factors that affect the accuracy and precision of this method include instrumentation, subject factors, technician skill, and the prediction equation formulated to estimate the Fat Free Mass. Criticism of this methodology is based on where the conductors are placed on the body; typically they are placed on the feet, with the current sent up one leg, across the abdomen and down the other leg. As technician error is minor, factors such as eating, drinking and exercising must be controlled since hydration level is an important source of error in determining the flow of the electrical current to estimate body fat. As men and women store fat differently around the abdomen and thigh region, the results can be less accurate as a measure of total body fat percentage. Another variable that can affect the amount of body fat this test measures is the amount of liquid an individual has consumed before the test. As electricity travels more easily through water, a person who has consumed a large amount of water before the test will measure as a lower body fat percentage. Less water will increase the percentage of body fat. Also reducing the reliability of this method is the variation between models of the BIA devices: for instance when comparing outputs from a Tanita scale to an Omron Body Logic handheld device the Tanita scale overestimated the percentage body fat in college-aged men by 40% and in college aged women by 55%^[9]. Bioelectrical impedance analysis is available in a laboratory, or for home use in the form of body fat scales and hand held body fat analyzers.

 Thanks.  I didn't feel anything, so didn't even realize that it was doing anything.  I don't think I put much stock in that stuff since I still have so much too lose.  If you aren't a body builder, does it really matter what your body/fat ratio is as long as you are at a comfortable weight for you and healthy?

To me... Body fat % is WAYYYYYY more important than weight! What if you GAIN 4 pounds but lose 2% of your body fat. Which would you rather have? I know what I would rather have...

 Good way to think about it.  

To add to that, the hand-held ones tend to over-emphasize the upper body while the scales that shoot the electricity through your feet over-emphasize the lower body. Depending on your body shape, this can throw off your results in a number of ways.

The best use of this particular measure is to monitor trends. So, you want the number to generally be going down and not worry so much over what the exact number is.

 Thanks.  I've never given it much thought, but I guess they care about it at the gym.

It's as good as any measurement while you are losing weight.  You want to see the body fat% drop.  At some point i think it gets less useful - i think excess skin confuses it all, but I liked seeing that I was burning fat and gaining muscle in numbers.