Just out of interest, here's a discussion among flight crews about automatic trimming versus doing it yourself. There's some jargon in there but you can probably follow the general line.

http://www.pprune.org/forums/archive...hp/t-9346.html

A "scarebus" by the way is Boeing pilotspeak for an Airbus !

Regarding the point about putting the seat belt sign on, if you think about it that is not the thing to do. Once the crew feel an imbalance which is significant, you do not want to freeze the passengers in such a position, you want it back as it should be. So a quick discussion on the interphone with the lead FA to ask what is going on, as crews cannot look back down the cabin nowadays, and then an announcement for passengers to please resume their original seats. It doesn't happen often because things do tend to balance out and there is in truth quite a margin. Incidentally there is trim both fore-and-aft, and also left-and-right.