Disgracin' The Basin

To clarify exactly what you need to do:

You need to use flags that are wholly owned by the opposite faction, or are neutral. A base that was previously yours but is now being converted does not count (as you are defending it, not assaulting it), but a base that was previously *neutral* (at the start of a match) and is being converted by the enemy does count.

Basically, when using a flag, if it makes you wait a minute before fully capturing the node (instead of turning instantly) then you've successfully assaulted a base. If you don't have to wait, you haven't assaulted.

Yes, it is pronounced bay-sin and not bass-in.

Just a guess, but maybe Qmccumber was referring to this quest - http://www.wowhead.com/quest=8120 You can get quest credit for the assaults if you are near a teammate capping a node. It doesn't towards the BG achievements of course. (Or maybe having this quest somehow bugged his ach., who knows!)