Mihaly Csikszentmihalyi devoted an entire book to Flow: The Psychology of Optimal Experience:
You have heard about how a musician loses herself in her music, how a painter becomes one with the process of painting. In work, sport, conversation or hobby, you have experienced, yourself, the suspension of time, the freedom of complete absorption in activity. This is "flow ." . . . (from Amazon.com, linked above)
Everybody has experienced a sense of “losing oneself” in an activity – being totally absorbed in a task, a movie or sex. Now researchers have caught the brain in the act.
Self-awareness, regarded as a key element of being human, is switched off when the brain needs to concentrate hard on a tricky task, found the neurobiologists from the Weizmann Institute of Science in Rehovot, Israel.
The team conducted a series of experiments to pinpoint the brain activity associated with introspection and that linked to sensory function. They found that the brain assumes a robotic functionality when it has to concentrate all its efforts on a difficult, timed task – only becoming "human" again when it has the luxury of time.
When an athlete says "I lost my concentration," he means that his state of "flow" was interrupted. In "flow" he doesn't actually "concentrate" (or think) about what he is doing. To the contrary, he simply lets his training and innate skill take over. But when his "concentration" is broken he becomes more aware of what he is doing, that is, self-conscious. And, in his self-consciousness, he does things that interfere with his performance.
I used "flow" when I was a student. I tried to deeply understand each subject (or at least those in which I was interested) by making the material "mine" through diagramming and outlining -- rather than mere memorization. Come exam time, I would spend my evenings at the movies and get plenty of sleep. If I had "crammed" it would have broken my "flow."