Flow states happen at the exact edge of your ability

Psychologist Mihaly Csikszentmihalyi identified 'flow' — a state of complete absorption and effortless performance — through interviews with surgeons, rock climbers, chess players, and factory workers. The key condition is a precise match between the challenge level of a task and the person's current skill: too easy causes boredom, too hard causes anxiety, but the precise edge produces flow. Brain imaging studies show flow involves reduced activity in the prefrontal cortex — the seat of self-monitoring — which explains why people in flow feel 'outside themselves' and lose track of time.

Source: Csikszentmihalyi, Flow: The Psychology of Optimal Experience (1990)