Super Smash Bros. has a concept quite different from most fighting games; rather than trying to deplete an enemy's life bar, you're out to increase their damage percentage and send them flying off the screen. The higher an enemy's damage percentage, the further they fly. And so it has been for generations.
In Smash Bros. on 3DS however, the attacker's damage percentage factors in too - the more damage you have when you hit your foe, the further they'll get knocked back (or up, or down, or sideways). It's unknown if the feature is something unique to the 3DS version of the game.
Just to be super clear here, this does not mean that the higher an attacker's damage, the more damage they'll do. The effect seems limited to knockback only. CLASH Tournaments, a Smash Bros. fan group, refers to it as "The Rage Effect." You can check out a video showing the new mechanic in action after the break.