When you use the powerful Up + B attack most characters will do an attack that will give them another extra boost. This is most effective after a jump and a double jump. Here is what happens.

Ness: Press Up + B to make him do PK Thunder. However, if you make it turn and hit him, he will get a very powerful boost in any direction. You will not get hurt.

Fox: Press Up + B and he will do a charge for Fire Fox. He can do it in any direction by pressing the Analog-stick when he charges.

Samus: Press Up + B to do a mid-air roll that does damage similar to Mario's Super Jump.

Pikachu: Press Up+B to do Agility. If you press Left or Right immediately after that, you will move twice.

Jigglypuff: Pressing Up + B does not do anything. However, if you jump, punch, float, punch, and continue in that pattern you can go very far distances (but not much in height).

Kirby: Press Up + B to make Kirby do his Final Cutter, making it go slightly higher for a split second. Do this only if you know that a quick, short boost will get you to where you want to go.

Mario: Press Up + B to have Mario do his Super Jump, giving him about as much boost as a regular jump would.

Luigi: Same as Mario.

Yoshi: None.

Captain Falcon: Press Up + B to have him do a spin in mid-air that can easily be controlled.

Donkey Kong: Press Up + B to have him do a helicopter attack that will boost him. This will not boost him if he is on the ground.

Link: Press Up + B and he will do a sword spin attack that boosts him. This will not boost him if he is touching the ground.