Flow, as conceived by Mihaly Csikszentmihalyi, is an optimal experience that is supposed to characterize activities that are "fun" or bring us pleasure. There are supposed to be "some ... activities that consistently produce flow," namely sports, games, art, and hobbies." Indeed, he argued that "the more a job inherently resembles a game - with variety, appropriate and flexible challenges, clear goals, and immediate feedback - the more enjoyable it will be."
Most importantly, “flow” is supposed to be a mental state of deep concentration that makes us more creative and more productive. But "flow" does not just happen. If we are talking about reading, studying or thinking, we need some time of uninterrupted activity to "get into" such a state of “flow.” Furthermore, interruptions or distractions are supposed to "break" this “flow” and make us less creative or productive, not to say anything about the pleasure brought by the state of flow. I think "concentration" is a more traditional word that covers a large part of the meaning of "flow."
Generally put, Csikszentmihalyi thinks that flow is a "state of inner experience in which there is order in consciousness." People who experience flow, also tend to say that their perception of time changes when they are in the state of flow. And we all would agree that "time flies when you are having fun." Klein (The Secret Pulse of Time. Making Sense of Life's Scarcest Commodity) argues that the "crucial element is not the activity per se, but the optimal density of information in the brain" (97). I don't know, whether this is correct nor even whether all that Csikszentmihalyi says about this "optimal experience" is correct. However, I would agree that there is certainly something to these observations.
There is an interesting Website that summarizes research into "flow" as it applies to note-taking. In particular, it talks about the kinds of notes we make to get things out of our minds so as to not to interrupt flow or to lose concentration. While it concentrates on a particular program, called "Notelens, it seems to me that the reflections apply also to other note-taking strategies that involve other means to minimize interruption. See
Flow and Notelens
In fact, you might say that the AutoHotKey Script I described in Self-Control is an attempt to achieve the same effect.