It looks like the app uses the built in HR monitor. Access to the HRV on the OHR built into the watches was added in CIQ v3, the FR735 is one of the devices which are staying at CIQ v2. Therefore no access to HRV data using the OHR.
Yes, as sabeard explained the app uses the built-in HR sensor on the watch. The app can work with all CIQ 3 compatible watches that can track Heart Beat Interval data. Currently the D2 Charly and D2 Delta Family crash on the simularor due to bug in Connect IQ so they are not supported at the moment.
Is it possible to get somehow results of HRV tests in Garmin overview - I mean they have their own HRV test/Stress Score, where chest strap must be used. HRV than can be shown as a long term overview. But obviously HRV Test results based on optical sensor are not paired with this statistic. Hope I describe that clearly. Another thing - how to interpret the results? I am not sure, what results I can read from measurement.
one282 , putting the app's results on Garmin's overview page is not possible. Only internal Garmin apps can be there. Until Garmin extend their API for 3rd party developers to put custom metrics on reports, none of the HRV metrics can be shown there. To interpret the results did you follow the measurments link on the app description? https://github.com/vtrifonov-esfiddle/TestHrv/blob/TestHrv-publish/README.md#measurements
