Down special move
From SmashWiki, the Super Smash Bros. wiki
Down specials are the most varied of special moves and are often of the character's more "gimmicky" attacks - the majority of reflectors, absorbers, counterattacks, item-creating moves, mid-match character transformations, difficult recoveries and chargeable attacks are down specials. They tend to involve minimal character movement.
Certain down specials do not function in the air at all, such as Peach's Vegetable and Donkey Kong's Hand Slap prior to SSB4. Others will display unique animations in the air but still fail to activate, like the first part of Villager's Timber.
Reverse down special
In Brawl, some characters' down special moves can be reversed by pushing the control stick or D-pad in the opposite direction a few frames after inputting the move. There is no animation for reversing a down special. Unlike other reverse specials, a reverse down special has a limited number of characters that can do it and fewer who will gain an advantage from it. The characters who can perform it are: Bowser, Yoshi, Pit, Ice Climbers, R.O.B., Pikachu, Marth, Ike, Ness, Lucas, Fox, Wolf, and Mr. Game and Watch. Since only Bowser, R.O.B., and Yoshi have a use for a reverse down special on the ground, some characters can use it in the air to gain a boost in the opposite direction and surprise opponents like Marth, Ike and Pikachu. The boost is most likely caused from a reverse down special having no animation and swaps the momentum around with the character.
List of down special moves
Ads keep SmashWiki independent and free :)