Oh, the way people sometimes talk about it, I really thought it must be some amazing feeling, and I'd definitely know if I'd experienced it! I'm slightly disappointed now.
I'm not sure either. I definitely remember occasionally feeling a distinct "flow" feeling the first few years I was studying and working. But now I still have periods of similar and superior concentration, but it's not accompanied by any extraordinary feeling.
I'm not even sure nowadays if I really experienced this, or if the feeling is just nostalgia.
