The heart rate sensor in Apple Watch uses what is known as
photoplethysmography. This technology, while difficult to
pronounce, is based on a very simple fact: Blood is red because it
reflects red light and absorbs green light. Apple Watch uses green
LED lights paired with light‑sensitive photodiodes to detect the
amount of blood flowing through your wrist at any given moment.
When your heart beats, the blood flow in your wrist — and the
green light absorption — is greater. Between beats, it’s less. By
flashing its LED lights hundreds of times per second, Apple Watch
can calculate the number of times the heart beats each minute —
your heart rate.