We use RR-intervals (the intervals between heartbeats) in order to estimate key indicators. You can read more about the process here. It means we use devices that read heartbeats and counts the time between beats.
All devices that measure HRV fall into one of the following categories:
Passive electric/electrocardiography/ECG – get an electric signal from the heart. All chest straps, like Polar and Garmin, belong to this category.
Studies that compare data obtained through electrocardiography and photoplethysmography show that both methods are suitable for measuring HRV. Plus, this study published by Elsevier proves that Welltory's HRV measurement accuracy is on par with ECG monitors.
Important note: although the electrocardiography technique is used for taking HRV measurements with chest straps, HRV measurement is not an electrocardiogram recording, and it can't be used as a substitute for ECG or a medical check-up.