I don't know about the ww scales, but I'm assuming you are talking about the body fat composition scales.
I found this on another website on how they work:
How a Body Fat/Water Scale Works
Using a technique called Bioelectrical Impedance Analysis, these scales send a small, harmless electrical current through your body when you step on the scale. This current passes more slowly through fat than muscle, giving you a measurement of how much fat is in your body compared to the muscle, or your body fat percentage.
I've personally never had any problems, and never heard of any problems. I do know they are not 100% accurate but give you a decent picture of your progress instead of just a number.
| Pounds lost: 9.4