Marth (SSBB)/Down aerial
Overview
Marth does a forward to backward downward sword slash. When tipped, it is a meteor smash with high base knockback. However, the sweetspot is significantly smaller than the sweetspot in Melee and as such the move is drastically less useful overall. The end of the swing is slightly easier to tip with, so a RAR may help sweetspot it. With very low start-up (6 frames), it is the second fastest meteor smash in the game (Falco's and Fox's down aerials are faster by one frame). It is a risky edgeguarding move, however, as it has very high ending and landing lag and the sweetspot cannot be consistently landed, especially on more competent players. Potential setups exist, but most are highly unreliable; one setup in particular, landing the sweetspot out of a grab release, is notable for being reliable and feasible in competitive play - on Meta Knight only. The move does 12% at the base and 14% when tipped.
Vertical spacing
Vertical spacing (using a footstool to setup the move) is also a method of sweetspotting the attack, though this is unreliable, because of the insanely difficult read needed to land the footstool.
Some characters are difficult or impossible to vertical space, as their falling speeds are too great. It is impossible to vertical space on Donkey Kong, Diddy Kong, Captain Falcon, Ike, Marth, Ivysaur, Ganondorf, Pikachu, Peach, Zelda, Sheik, Sonic, and Wario. However, it still works by footstool jump 2 times or more for a tipped down aerial to hit. It works best on small, light characters, such as Kirby, Meta Knight, and Jigglypuff, but even Bowser can be vertical spaced with little difficulty.
After testing, it has been confirmed that the combo can be used on certain characters, such as Link or Toon Link during their up special move.
Toon Link is able to meteor smash with this technique, although his does not blend fluently like Marth's. It is about stunning the opponent to set up for the spike frames at the beginning of Toon Link's down aerial.
Meta Knight is also able to perform a footstool jump to down aerial combo, but instead of spiking, the opponent is semi-spiked away; in most situations, the victims are either sent into the stage and are either stage spiked or sent away from it.
Vertical spacing tutorial video
<youtube>mjKG4amJJ6E</youtube>