"Falling of the bottom" refers to the situation where the rpm falls below the level at which the engine is self sustaining. This can happen following a rapid throttle back if the airflow conditions around the engine are abnormal or if there is a minor fault somewhere. If you develop your own ecu (as I have) it is a problem that you become aware of when you are trying to optimise the throttle response.
Gaspar Espiell (who designed the Jets Munt restart system) gives the reasons here
http://www.jetsmunt.com/wp/2016/07/1...tart-function/
It follows my thinking pretty closely - I can't do much better - except to add that 15 seconds is usually plenty of time for a safe deadstick landing.