It inflates the wristband and uses oscillometric measurements to estimate the BP. Source: the user manual (linked on the site near the bottom of the page). It'

It inflates the wristband and uses oscillometric measurements to estimate the BP. Source: the user manual (linked on the site near the bottom of the page). It's the same way most home blood pressure monitors work. It's not as accurate as good old mercury+stethoscope, but it's pretty decent. Generally speaking, measuring on the wrist further reduces accuracy. The absolute values might not be the most accurate, but I suspect the trend over months/years will be accurate enough.