MedWatch uses Pulse transit time technology (PTT). It provides a basis for ubiquitous blood pressure (BP) monitoring. PTT is the time delay for the pressure wave to travel between two arterial sites and can be estimated simply from the relative timing between proximal and distal arterial waveforms.
The faster that pulse transit time is, the more the vessels are tightening — that's what's making the pulse travel faster. Optical sensors also check if vessels are tightening or widening. An algorithm then uses those two bits of information, along with heart rate, to estimate blood pressure.