The underlying technology used by the software is called Eulerian Video Magnification (EVM), which essentially tracks the variation of individual pixels over time — and then exaggerates those differences. As your heart pumps blood around your body your arteries swell with bright red blood, which changes the color of your skin slightly. To the human eye, no matter how long and hard you stare at your wrist or someone else’s face, you would struggle to detect a change in color (unless they blush, of course). For a computer, however, the tiniest per-pixel fluctuations (between white and slightly-redder-white, say) are easy to detect. In the case of detecting someone’s heartbeat, EVM picks up these slightly redder pixels and exaggerates them, turning them violet.