Flow is…
“…being completely involved in an activity for its own sake. The ego falls away. Time flies. Every action, movement, and thought follows inevitably from the previous one, like playing jazz. Your whole being is involved, and you’re using your skills to the utmost.”
-- Mihaly Csikszentmihalyi
One of the pillars of Flow is controlling our consciousness and how we perceive what happens in the real world.
To do that (while not becoming delusional), you need to clearly know both your conscious mind and the external world. If either is fuzzy, you are in danger of going off track and never achieving Flow and happiness.
When dealing with your idea, and its implementation, you need to do the same. Know exactly what the essence of your idea is. Have the right way of looking at the real world, the results of your actions, and of implementing your idea. Understand how to judge them.
You need to keep both aspects on par. If you neglect the reality of your implementation, you’ll end up delusional. Neglecting your idea will lead to stagnation.
But Flow cannot be a constant state.
As you advance towards your goal, you will concentrate more on one than the other. It will be either the real world (implementing your idea) or the idea itself. Concentrating more on implementation can lead you off-track and into stagnation.
Too much emphasis on your idea and you might disconnect from reality and be in danger of becoming delusional.
To keep on track and in Flow, you need to “up your game” by keeping both your idea and your implementation in mind. The tension between idea and implementation will lead you to Flow with your idea.