@ Piper Classique.... I am not familiar with the technical side of "shower of sparks" magneto, but it's very simple to do with coil ignition. The points could operate a transistor to hold an astable oscillator "off"...the points open, transistor switches oscillator "on" oscillator , in turn, feeds it's output pulses through the primary of an ignition-coil thus you get a shower of sparks which switches off when the points close again.
As a kid, I had a "shock-coil" -basically it's a transformer, (as is an ignition coil)
the primary connected in series with a buzzer/electric bell....connect to a battery and it operates as follows...
solenoid coil of buzzer/bell and primary of HT coil (series!) energise...solenoid pulls spring-loaded armature (a bit of soft iron) towards it...the primary circuit is arranced so a break in the wire is connected one side to a fixed contact point and the wire continues from a contact -point attached to the armature... thus, when the armature moves towards the coil, it breaks the circuit, the magnetic field collapses and the armature springs back to complete the circuit again...
Distance from coil to armature, strength of spring (and one or both contacts can be sprung as well!) and supply voltage all affect the rate of oscillation....
Meanwhile, the pulsing on-off current is passing through the shock/spark coil primary winding.... every time the coil is energised, it magnetises it's core and every time the current stops, the magnetic flux breaks down and induces a voltage into the secondary.
this is necessarily a shortened explanation, i've deliberately left out the stuff
Phiggs alluded to, such as quenching capacitor across the points.
I suppose a magneto could be arranged with a modified form of impulse-coupling, such that when the coupling "flicks" the mag, the rotating part of the Mag has a one-way clutch with a single engagement-point and sufficient mass that the impulse is able to spin the armature multiple times until it slows sufficiently for the drive to "lock " again that would give a brief shower of sparks, but remember the Mag has a permanent magnet and the armature is "braked2 every time it's poles pass the poles of the field magnet.
Physics....the energy for the spark has to come from somewhere.
I must start going out!
Forgot to say, an electronic ignition system can trigger a shower of sparks and , to prolong plug-life, revert to single-spark under benign running conditions.
@Phiggs...yep, I had an air-cooled single "Tuff-Tuff " with a Wipac impulse mag....hand cranked it would "clink....clink.....clink as the impulse fired....if you cranked hard enough, you could exceed the impulse cutout speed
just pulling the flywheel to the "Clink" would often set it going.
we used to drag it out and set it going on the workshop floor...as it had a speed-govenor, it would scuttle about as you revved it, and judicious jiggling of the throttle would produce backfires of vivid blue flame from the exhaust stub....a length of pipe could be held with one end near the stub and be "tuned" like a trombone
Amazingly, I found time to work as well! Think it was a couple of quid, non-running at an auction. best value entertainment ever.