Flow is the state psychologist Mihaly Csikszentmihalyi named: total absorption in a task that is neither too easy nor too hard for your current skill.
Playing at the edge of your ability is exactly what builds ability — so the right difficulty is a moving target. It has to rise as the player improves. That's the gold channel climbing under your curve.
Too far below skill and the player is bored; too far above and they're anxious. A difficulty curve threads the rising channel between the two.
Frustration builds fast but fades slowly. A single spike that outruns a player's skill can sour the next several minutes and end the session — which is why designers use plateaus: brief flat stretches that let mastery catch up before the next climb.
This lab hit a snag — the lesson continues without it.