Hi, we once had a Petter stationary engine, on a water pump, that had an Impulse Magneto. After it had run for a bit to get it warm, if we needed to re-start it, all we had to do was crank the flywheel over slowly (even taking 10 seconds from BDC to TDC.) and when it reached TDC the Impulse would occur, and the engine would thump into life.
The voltage produced by a coil with a contact breaker is proportional to the rate of change of current (di/dt.). So if the points open instantaneously, there is a very large voltage or back-emf produced.
(I have forgot the actual formula but suspect it is something like
dv = L di/dt where L is the Inductance.)