system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how pulse detection apps work

I have a heart rate monitor app on my phone which works pretty gosh darn well.  I place my finger over my phone's camera and LED flash and it accurately measures my heart rate in real time. 

My nurse girlfriend has taken my actual pulse at the same time and it's always very close. 

So how does it work?
gpt
Try this: open the camera app on your phone and start recording video while turning the flashlight (LED lamp) on, and then press your finger on the camera lens. You will see a blurred image of your finger and if you look closer you'll see a slight pulse in the color. Thats your heart beating. 

It works because the flashlight will penetrate your skin and scatter all the way around in your finger including your artery, and some of the light getting back to the camera gets slightly dimmer when your heart pumps.