Visible armband

What exactly does the Visible armband do, and how does it work/do you think it’s worth buying? I suck at constantly pushing myself too hard; I don’t even realize how bad I feel until after the fact because I mask it so well. If it’s important, the conditions I would want it to help track are POTS, hEDS, migraines, and/or depression. From what I’m reading about it, it gives you alerts when you’re pushing yourself too hard, but how does it determine your limits?

I just want to see if it’s forth it to subscribe to it with my limited resources.