I recently got the Heiress champion.  Her passive says that she "Counterattacks the attacker when an ally is attacked."

However, this never seems to trigger when an ally is attacked.  It only seems to trigger some of the time when she's attacked.

Two questions:

1) Is the description wrong?  Should it be "Counterattacks the attacker when this champion is attacked"?

2) Do passive skills have a cooldown?  Is she only able to counterattack once per turn (and that's why it seems to be only triggering some of the time)?

EDIT: Ok, well now after ascending her to L3, she's counterattacking when allies are attacked too.  But, there still seems to be times when she doesn't counterattack, like there's a probability associated with triggering her counterattack (or only once per turn)?