I don't know about the physiology behind it, but some personal observations after 18 months of HR monitoring.
When I go into PEM my resting HR goes up (can be by as much as 35bpm), and my upright tachycardia is worse.
But there is a point when I've overdone it too badly, where although my resting HR stays high, my upright HR doesn't rise as much as usual.
Although my HR looks more 'normal', I usually feel much worse.
When I was going for my Tilt Table Test, I was concerned about the effort of getting to the hospital for the appointment would push me into that second stage.
It did push my resting HR up to 90bpm at the hospital and even though they monitor you lying down for a while before starting the tilt it didn't drop.
My morning resting HR in bed had been 69, but I still managed to fit the POTS criteria from 90bpm.
I can't really use HR monitoring as described by Workwell and others, as I'd rarely be able to do anything upright and stay under my estimated threshold. Use it more just for feedback on what my tachycardia is doing.
When my resting HR is in the low 60s it is a sign that I'm doing better with pacing.