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I recently got sent the updated HRV manual from @EliteHRV. If you have recently upgraded their app then you might want to take a look as the content is greatly increased.
If you don’t know anything about HRV and you have 15 or so minutes to spare then downoad the Elite-HRV-Manual and have a good read, it will give you an idea of the sort of things possible from HRV. Elite HRV is one of the more comprehensive apps of its type, although I don’t use it myself.