Megan.
I think the answer is that no one knows. We can look at rules and SOPS but practically as they stand it doesn't work.
If its when the doors close, then for example if there is an APU fire during the transit,who makes the decision to carry out the procedure? In my old company that actually became a major argument after the FE who was on board when the fire alarm went of fired the bottle only to have the LAME complain as the engineering handover wasn't complete. The argument was that until the M/R is signed engineering still has authority. Yet in that situation Engineering had no training in evacuating passengers.
In another case after engineering calling a brake fire after parking, an overzealous Policemen detained the Captain before he handed over to the Fire Commander. Who was then in charge of the operation at that time?
To a degree this is a bit like the Gimli episode where the FE-SO was replaced without anyone looking at who would carry out the abnormal fuelling ops. It seems to me that we have moved well away from the old model of Legacy Airlines with their own well qualified staff and replaced them with poorly trained contractors, without adequately examining everything that this now replaced staff did.?
Hopefully this whole shameful episode will at least start such an examination throughout the Industry. But since it will if properly done throw up numerous costs I somehow doubt anything will happen.
Wunwing