This spell works like an interrupt, where it is best used during an enemy's spell cast. Miscast Magic has the unique ability to cause an enemy spell to backfire on them drastically. If used properly, a mages fireball will end up blasting the mage or one of his allies, instead of you. An enemy healer's spell will end up healing one of your party members, instead of one of theirs. A warlock's fear would go awry, fearing one of his own team-mates, instead of yours.
I don't think this mechanic is in the game yet either. I'm just being creative and trying to add some flavor to the spec. Thoughts?