Some unscrupulous OEMs have been including fake heart sensors in their cheap fitness trackers
There are tons of cheap smartwatches and fitness trackers on retailers like Amazon, AliExpress and so forth. Many examples have advanced features too, like heart rate monitoring. Heart rate monitors in smartwatches and fitness trackers generally use photoplethysmography to detect heart rate changes. Typically, OEMs use green LEDs to measure heart rates, although some devices like the Apple Watch also use IR when they are measuring your heart rate more infrequently.
However, it has emerged that some cheap fitness trackers that claim to support heart rate monitoring do not contain a heart rate sensor at all. Instead, a few OEMs have been including boards with two green LEDs that mimic the flashing behaviour of photoelectric heart rate sensors.
CNX Software opines that these devices, an example of which the website has given, use their accelerometer to estimate heart rates. It is unclear how many other fitness trackers have been equipped with fake heart rate monitors, or whether the practice also extends to smartwatches. CNX Software warns that many claim to use the Telic TLSR8266, though.