I think that the mechanical effect is the aspect that's already on them. You could potentially tag the "Human Shield" aspect for effect to say that the next attack made against you hits your human shield instead, if your GM allows for it. And after you've thrown your "Human Bowling Ball", you can spend a FP to get a +2 on an attack against that enemy, with the justification that the enemy just got flung around and is probably too disoriented to get out of the way of your attack. Or, if your GM is willing, spend a FP to tag it for the effect that your enemy lies there groaning for the next exchange, or that said enemy has the "Bruised" aspect now, etc.