I stand corrected, it is called the Omron Karada Scan. It basically passes a weak electrical impulse through your body and determines its composition based on that.

"Muscles, blood, bones and body tissues with high water content conduct electricity easily. On the other hand, body fat does not store much water, therefore has little electric conductivity." ~ According to the User Manual (https://omronhealthcare.com/wp-content/uploads/hbf-510w-instruction-manual.pdf)