For me there are two big contributors to flow (and flow may not be the same for everyone): requirements are very well understood and willingness to code the req

For me there are two big contributors to flow (and flow may not be the same for everyone): requirements are very well understood and willingness to code the requirements. If it's boring or boilerplate it doesn't work. Otherwise, some kind of flow normally happens. I believe you require some amount of concentration hence the need to not be interrupted, and it has to be a bit challenging to require concentration. An example would be an interview or college exam: you are pressured and needed to stay focused, yet the time passed very quickly.