Are they using the method called flash photoplethysmography? Or are there other sensors involved? What factors affect the accuracy rate? What should we pay attention to during use?
How do smartwatches measure heart rate?
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Yeah, most smartwatches use flash photoplethysmography (PPG), measuring heartbeats by reflecting light onto the skin. My Garmin watch uses this same method too — the sensor on the wrist shines green light onto the skin in pulses and detects changes in blood flow. The accuracy is generally good, but it can cause errors when you're moving around or if the watch band is too tight or too loose.
The times it's most off for me are during exercise or when I've worn the band too loose on my wrist, since the sensor isn't making proper contact with the skin. Also, since light absorption differs with darker or more pigmented skin tones, the readings can get skewed. When I'm using it, I make sure my wrist is clean and dry, and I double-check that the sensor sits flush against my skin.
Besides flash photoplethysmography (PPG), some smartwatches also measure heart rate directly using an ECG sensor, like the Apple Watch's ECG mode. While PPG emits light and detects blood flow, an ECG reads electrical signals, so it provides more accurate results. Factors affecting accuracy include the sensor's contact with the skin, motion sensors, and not keeping your arm still during the pulse measurement.