I suspect "in the zone" is the better-recognized phrase, despite the book.

Fair enough! Perhaps "also known as..." would have been better.

Long time practitioner of this when running as well as coding, I only recently found out about the buddhist Jhāna and Pīti a.k.a "joy", "rapture", and "ecstasy" in their more western specific spiritual meaning. Also ties in with hyperfocus. This whole area is fascinating and seems to have a common underlying process but I couldn't really find anything besides my own intuition of it.

https://en.wikipedia.org/wiki/Pīti

…or Focused.