Superposition

A qubit can hold a combination of 0 and 1 at once — a superposition. What it holds are amplitudes, not probabilities: complex numbers that can add or cancel. The probabilities appear only when you measure, as the squared magnitudes.

That cancellation is why a circuit can do work: gates arrange amplitudes so that wrong answers cancel and right answers add. Noise disturbs the arrangement. Past the edge, the arrangement is gone and every answer is as likely as any other — the coin flip on the ratio curve.