The blood pressure and heart rate monitor is based on the latest technology called photoplethysmography. Simply put it is the use of green LED lights paired with light‑sensitive photodiodes to illuminate your skin and measures changes in light absorption.
This lets the device detect the amount of blood flowing through your wrist at any given moment. Because blood is red, it reflects red light and absorbs green light.
“When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less,”.
The sensor are designed to compensate for low signal levels by increasing both LED brightness and sampling rate. That’s why the heart rate sensor on the back of the device flashes its LED lights hundreds of times per second, helping the device calculate heart rates precisely.
But when it measures your heart rate every ten minutes, the Watch switches to using infrared light. And if it fails to provide an adequate reading while using the infrared light, the device switches back to the green LEDs.
In order for the sensor to work as advertised, the watch must be close to your skin. The company advises tightening the band in case the sensors aren’t reading your heart rate accurately.
All in all, the heart rate sensor in the watch is a great feature, especially if you’re something of a fitness buff. It checks your heart rate during workouts for an at-a-glance overview of both the intensity level and the heart rate change over time.
For detailed reading on photoplethysmography & how it's used in "measuring oxygen saturation, blood pressure and cardiac output," check this website out https://www.ncbi.nlm.nih.gov/pubmed/17322588