They say they measure body fat and hydration, but really they measure water and call it two different things.
Muscle is wet and fat is dry. These scales send a very tiny electrical current and measure how fast it goes around and comes back. Since water is a good conductor, if you have a lot of water in your body, it will move faster than if you're drier. Then the computer in the scale uses a formula to make a guesstimate about how much of that water is in lean muscle tissue and how much is in other tissue, and gives you a guess at your fat percentage.
It's not accurate, but that doesn't mean it's not useful. It's great for your psychological state because it can explain weird sudden weight gains. If your weight goes up but your "fat percentage" goes down, it means you're retaining water and you probably don't have anything to worry about. If your weight and your "fat percentage" both go up, you probably have actually gained some fat and you need to figure out why. If your weight goes down but your fat percentage goes up, you're probably a bit dehydrated. If they both go down, congratulations! The weight loss is probably really fat loss.
It really just identifies trends, though. It's not good for knowing what your actual body fat percentage is. If it gives you a reading of 30% on average in March and 26% on average in August, you probably really have gained some muscle/lost some fat. You might really have 26%, but it's just as likely 32% or 21%-- but it's less than it was in March. That's all you'll know, but that's good information to have.
I've had my "body composition" scale for about 8 years now, and even though I know, intellectually, whether a weight change is real or water, I still really like having the confirmation of that second number. I'm not sure I would encourage someone to buy one if it was a lot more expensive than a regular digital scale, but since you've already got it, you'll probably like having it to see which way you're going in general over time.
| current weight: 132.0