I believe Rejzor described it quite well in some post.
The "emergency update" is as simple as possible, it has nothing to do with the normal update process, it's not really an update. It basically only downloads a file from our server (if any is available - which is normally not the case).
So if this "emergency scenario" occurred, we would have to prepare some fix - a program that would fix the avast! installation. This is just a way of distributing it without the need of user's intervention, nothing else.