Pretty neat how it works. The watch will use the same technology that the ECG will use (electrical signal) and the same technology for PulesOX (blood flow). It looks at the difference between the electrical pulse and the subsequent flow of blood to determine the pressure. This is why it needs to be calibrated- everyone will have slightly different arm lengths or other irregularities. The calibration tells the watch how to calculate the differences, and then spits out and amazingly accurate blood pressure number.