They're out there. In addition to the impuse system, there was a
starting
system called "shower of sparks" that used a gadget called a vibrator to
provide the low rpm spark. Unlike the impulse system, the vibrator
required
electrical power from the battery to make a spark.

I've usually seen these on older models.

Yup, you're right. Bendix made that system to provide a hot
spark for starting. The mag switch had extra terminals to provide an
AC signal from a buzzbox to the mag, but that mag had a second set of
points that gave the later spark for starting. Unison has a similar
setup now, but solid-state instead of an electromechanical buzzer to
create the signal for the primary winding.

Both of those spark vibrators just make a constant sparking, just waiting
for the right combination of fuel, air and compression, don't they?

If that is the case, it would seem like that would be like a very advanced
spark, and could easily make a backfire to spin the engine backwards. Is
this what you have observed?

If that is not how they work, then...
Never Mind ! ! ! g
--
Jim in NC