A pulse oximeter clips on to a fingertip, shines light through it, and watches how much light comes out the other side. With every heartbeat a little more blood arrives in the finger, the blood absorbs a little more of the light, and the brightness dips. The dips mark the beats. This is photoplethysmography (PPG), the same principle as the heart-rate sensor on the back of a smartwatch, and a phone camera can do it too: the lens looks at the fingertip, the torch lights it, and the app reads the brightness of the picture, frame by frame. From the beats it computes heart rate, inter-beat intervals, and a rough heart-rate variability. A music researcher who needs reliable beat timing uses a chest strap or an ECG, as described in the physiology chapter; this app shows what the optical principle can and cannot do.
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Things to try: sit quietly for a minute and note the rate, then stand up and watch it climb before it settles again. Put on a favourite piece of music and watch whether the rate moves, and when. Count your pulse by hand for 30 s, as in the breath and pulse app, and compare the doubled count with what the camera says. Finally, breathe slowly and deeply at about 6 breaths per minute and look for the intervals lengthening on the out-breath and shortening on the in-breath, which is respiratory sinus arrhythmia.
Limits: movement, changing light and a cold fingertip all break the measurement, and face mode breaks with far less. The app is a demonstration of a measurement principle, not a medical device. Do not use it to make decisions about your health, and compare it with a proper sensor before trusting it in a study.