User blog comment:Zolnir/RWBY Episode 6 Review/@comment-13616152-20130823142330/@comment-10390252-20130823143339

Simple: Normally she can't. It's another Aura effect.

It's now been establisehd that Aura power can amplify your natural abilities. So, consider the situation: Weiss has just delivered a fairly devastating one-line critique (and, let's face it, one that might match Ruby's own fears) of Ruby's worth as a Huntress after she screwed up what should have been an easy battle because she couldn't co-ordinate with Weiss properly. At that moment of anger, at Weiss and herself, she put all of her force, physical and metaphysical, behind that blow. It was several orders of magnitude stronger than her normal strongest attack.

If she ever develops the ability to consciously summon that power, I'm pretty sure that she'd be able to take down even massive foes with a single hit. At the moment though, she can't do that consciously and I'd bet she won't even be aware of that potential power until one of the tutors shows her the video in the post-test debriefing.