It's just like the Tanita scales, only that they are handheld. They both use Bioelectric Impedance Analysis, they send a weak current through your body and because fat and lean mass have different conductivities they can estimate BF percentage. If you use them at the same time of day, same level of hydration, sleep, exercise, etc they can be consistent. As for accurate, it depends on the person. They estimate total bodyfat and you have to also enter your age. This is because as we age we generally store more visceral fat. But it is the subcutaneous fat that determines how lean you look. That is, if two identical persons of 20 & 35 years are both 10%, chances are that the 35 yo will look more cut since he will have less subcutaneous fat. Calipers measure directly this subcutaneous fat, while BIA devices estimate both subcutaneous and visceral fat.