The knockdown stance rule state:
The barbarian can focus on toppling her foes. Once per round, she can make a trip attack against one target in place of a melee attack. If she succeeds, the target is knocked prone. This trip attempt doesn't provoke an attack of opportunity. This is a stance rage power.
Can this once per round attack be an attack of opportunity? I'm inclined to think 'yes', since it's a stance and you would normally be able to make a trip attack as an AOO, but that may simply be wishful thinking on my part.
