Basically you sense RPM by looking at a plug wire, loosely coupled as in video,
or the coil. If you turn off ignition the bike would be herky jerky, not a nice
ride by any means. Unlike what happens when you close down a throttle and
bike slowly loses speed.
A "smart" approach would be a combination of throttle and ignition, or just
throttle.
You might visit some bike dealers and see what they say about products
and how they are done and characteristics of various methods before you
jump off a cliff. I think of a bike as something to be careful about implementing
controls like this, given their stability a little less than cars, as example.
If bike injected one could alter the pulse going to injector reducing, but not eliminating,
fuel, a "fine" control, like throttle.
An Arduino, counting pulses at the higher RPMs, might be a little challenged to manage
controls. Experimentation might be best vehicle to evaluate this.
As aside I think of ignition cutout a good method of managing emergency issues, like
excessive RPM, as when a throttle is wide open and tire load, engine load, evaporates
(airborne....) causing excess RPM to occur due to inertia of the accelerating mass and
engine load.
Regards, Dana.