It increases it by 1. This was verified by parses on live shortly before they realized it was bugged and changed the AA on live.
EDIT: It looks like I never fixed this ... Could have sworn I did ...
NVM, I did fix it. You can find dev quotes on live forums with old formula.
https://github.com/EQEmu/Server/blob...tack.cpp#L2878 I even left a nice comment here :3