Vi measures heart rate using an optical heart rate sensor in the left earbud. The sensor emits a green light in a specific wavelength to the tissue of the tragus in the ear. The reflective light is captured by the receiver of the sensor -- this is then processed by a proprietary algorithm to capture the heart rate.
If you need assistance getting an accurate and consistent heart rate, you'll likely need to adjust your fit. Check out this tutorial video to find the correct fit!
Fun Fact: this sensor was originally designed for the air force and astronauts so it is incredibly accurate!