Wearables with optical heart-rate monitors have small LEDs on their undersides that shine green light onto the skin on your wrist. The different wavelengths of light from these optical emitters interact differently with the blood flowing through your wrist. When that light refracts (or reflects) off your flowing blood, another sensor in the wearable captures that information. That data can then be processed, along with motion information detected by the device's accelerometer, with algorithms to produce understandable pulse readings.
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Since most modern fitness trackers are wristbands, optical heart-rate monitors typically sit on your wrist. But we have seen some variation recently, like with [Moov's latest fitness tracker](https://arstechnica.com/gadgets/2017/02/moov-hr-review-everyone-should-wear-heart-rate-monitors-on-their-heads/) that puts an optical heart-rate monitor on your temple. [Earbud heart-rate monitors](https://arstechnica.com/gadgets/2016/12/hear-the-pulse-heart-rate-monitoring-fitness-earbuds-tested/) that also use photoplethysmography have started to become more common as well. No matter where the device sits, the PPG process is the same: measuring heart rate through analyzing light refracted off flowing blood.
Heart rate monitoring watches tend to have 3 bright LED's in a triangle. What do they do, how do they work?
https://arstechnica.com/gadgets/2017/04/how-wearable-heart-rate-monitors-work-and-which-is-best-for-you/#:~:text=Wearables%20with%20optical%20heart%2Drate,blood%20flowing%20through%20your%20wrist.