I’m assuming you’re referring to those electrical current ones. As advertised, the scale puts a small electrical current through your body from one end to the other and measures the resistance the current encounters. From here, along with your height and weight, the scale calculates your body fat percentage.

These scales are reasonable accurate. However don’t rely on the percentage if you are overly hydrated of dehydrated. As the scale sends an electrical current through your body, excessive water intake (or lack of) in a day can really throw the scale off.