I've been using the mobile device made by Heartmath lately — https://store.heartmath.com/tech/ . It looks at HRV (heart rate variability) and gives you a realtime plot, thereby offering visual feedback about how my breathing is affecting how steady the time between heartbeats is. Hearthmath calls low HRV "coherence" and getting into a lower HRV state (i.e. via regular, smooth breathing; maybe 1 in/out breath each ~10…
Then I gave it to my dad, who has been an ER doctor for 40 years but has no knowledge of or experience with meditation. Within seconds, his calmness graph plummeted to the minimum value (lower = better) and flatlined. There was a brief spike in attention when the dishwasher changed cycles in the other room, then it cratered again. He just nonchalantly explained that he can control his heart rate to calm himself down, which sounds like a superpower to me.