I’ve tried Biofeedback and while I’ve never heard the term heartmath before, after some googling, I recognized it. It clips on to your ear and measures your heart rate. I don’t think there’s a big difference between Biofeedback therapy with a heartmath device and Biofeedback without one. I used it in addition to other sensors (one for skin temperature, one around your waist to measure your diaphragm breathing, one for your galvanic skin response (sweat gland activity) and one for muscle tension).
What the heartmath device onscreen looked like was (https://static1.squarespace.com/static/55832ca1e4b047816d3d9c8b/t/575f1f578259b52cea31cb44/1465860311491/heart-math-emwave-biofeedback-device-focus-stress) and my goal was to make the waves as even as possible with six to a screen (through deep measured diaphragm breathing) and to get the green bar as high as possible. It’s been a couple years, but I’m pretty sure the red bar corresponded to adrenaline and the blue bar serotonin, so you’re trying to be relaxed, but not sleepy. I’m completely spacing on what the bottom-left corner picture means, I think it had something to do with concentration, as I said: its been a while.
I hope this helps, and I hope biofeedback helps you!
Let us know how it works out,