If FAS points exceeded, all the fleet enters battle. If you check with FAS, you have a certain part of the fleet covered and some uncovered. But if someone attack you, you will loose all the fleet, including the part of the fleet witch was supposed to be safe, because it is protected by FAS.
For example:
http://xcraft.net/battle/index.php?battle_id=ba461db48c0ae141962876ddd619142d.
All the fleet, excepting the death stars was covered by FAS, but, as you can see, I loose it all.
That's bull