This trick allows you to endure a hit that usually causes the knockback effects when using Ike. When you are about to be hit by an objects such as a Smart Bomb, press Up + B to do Ike's special move, Aether. Your sword must still be alone in mid-air when hit. If done correctly, you will continue to jump upwards toward your sword after the hit has landed and finish the move. This is very useful and can be much easier to time and execute than sidestepping.
