Yeah I believe if you parry it will allow you to use the skill.  Not 100% sure of this, but it is definately triggered by some kind of evasive maneuver.  (I know it's not dodge)

Edit:  Yes, I have watched closely and this skill is triggered by a parry. 

From my gameplay, this skill can be used at any time, but the use of the skill itself is somewhat indirect.

I can't refer to the tooltip at the moment, but basically this skill is designed to reduce the aggro of another person.  So, to use this skill you must target someone else and make them your defensive target (blue targetting circle) and then you can fire off this skill, reducing aggro from their aggro table and drawing it onto you.

The point of the skill is to make the monster hate someone a little less and you a little more.  It's the opposite of the goading Goading Slap skill.