For those of us who don't have a Polar strap, can you explain at a high level how your app works? Based on what the page says, seems like something about using R-R interval to estimate where you area on the Meyer wave cycle?

I have a different heart rate monitor (Amazfit smartwatch, mine has their latest sensor that matches the higher end Garmin watches for accuracy, it can be used as a Bluetooth device or you can develop software to run on it directly). What topics/keywords should I look into if I want to develop the equivalent application for my hardware?