PPG sensors measure pulse rate variability (PRV) using LEDs to detect blood volume changes, unlike HRV which uses electrical signals.

“they don't get the same electrical signal they're measuring changes in blood volume through the skin basically electrod shine electric or shine the LED light down into the skin it reflects differently based on the blood flow flowing through the the arteries below it and so you're getting the pulse and they actually call it pulse rate variability it's not really heart rate variability if we want to get technical it's pulse rate variability but it's showing us the same thing it's showing us that cardiac cycle”