The human heart generates a blood volume pulse (BVP) with each beat and is the source of the blood circulation. The resulting blood flow through the circulatory system leads to a continuous change in skin color. This effect can be observed more strongly when a person is physically active, e.g., after climbing stairs. The heart rate rises and the face color is getting redder. However, this constant color change is mostly imperceptible for the human eye. With rPPG techniques, this color variation is detected from a video, and the pulse rate can be determined.
This isn't some Intel-exclusive wizardry. It's just analyzing changes in the red color channel over time:
https://www.hhi.fraunhofer.de/en/departments/vit/research-groups/computer-vision-graphics/research-topics/video-based-blood-flow-analysis.html