It's a weird, full-body change for me. When I'm in that kind of zone it's like my pre-frontal cortex finally took over control from my lizard brain. Or maybe it's just hormones.
I can do what I choose to do. To focus. To exercise and give 100%. Uncomfortable sensations like cold weather or spicy food feel easy to tolerate.
One other anecdote: occasionally I will have a day full of energy and getting motivated to work hard on things comes easily. Almost like clockwork, I will realize the next day that I'm sick (like a mild cold). I'm leaning to the reverse causation hypothesis (as the body is getting sick, it provides me the energy to work hard to prepare for the later downtime) but I guess it's also possible that whatever energy I'm feeling also leads to immune system weakness?
Anyways, just sharing some thoughts. If I could figure out how to live at that kind of level even one day a week reliably, that would be incredible.