Final answer:
The breaker points in a magneto begin to open when the rotating magnet causes the magnetic flux to change from maximum to zero, inducing a peak emf in the coil.
Step-by-step explanation:
When internally timing a magneto, the breaker points begin to open when the rotating magnet is at the position where the magnetic flux changes from its maximum to zero. At this point, the induced electromotive force (emf) is at a maximum in the coil, which happens as the magnet completes a quarter of a revolution. This is due to the change in magnetic flux through the coil, which induces the emf according to Faraday's Law of electromagnetic induction. The timing of the breaker points is crucial to ensure the magneto generates a strong spark at the correct moment for engine ignition.