Kinda funny, I think there was a conference on this stuff a few months ago as I've seen a lot of stuff about HRV go by in my RSS reader lately.

I have a github project

https://github.com/paulhoule/VulpesVision

that does an HRV visualization on a web site if you (i) use Chrome and (ii) have a modern BTLE HR monitor (I got two Polar H10s) You can bring it up with

  npm install
  npm run dev

There is a lot of confusion in the literature I think, but you can clear it with the understanding that: (a) the SD1 metric aka RMSSD is the one that's the most backed by the literature and (b) you can increase your SD1 by reinforcing

https://en.wikipedia.org/wiki/Mayer_waves

by the simple method of eyeballing the instantaneous HR chart and breathing out when the chart goes down and breathing out when it goes in, being careful to manage your air supply (not breathe too deeply) so you don't top or bottom out.

Elite HRV is a pretty good commercial app but their "HRV biofeedback" isn't anywhere near as good as mine as it tells you to do "resonant breathing" by sweeping through the frequency range where the Mayer wave happens -- why do that when you can just sync it directly... in fact I can do the same feat eyeballing the heart rate in Elite HRV.

I'm working on a 2-player version of this game because I think it will make a demo that punches well above its weight.

I love tracking my HRV. It definitely follows my perception of stress, but watching it carefully has taught me a lot about what causes me to be stressed and what doesn’t. I love talking to people about this.

BTW, a really key tip: if you tell your Apple Watch you have AFib, it will take many more measurements of your HRV, making the value much more accurate.