Here on Smashtopia, some users like to include what we call the Extra Skill as a new form of Special Attack for playable characters.
Thus far, every Smash Bros. game (with exception to Ultimate) has introduced a new way to attack with Melee giving us Side Specials, Brawl giving us Final Smashes, and Smash 4 giving us Custom Moves. The Extra Skill is only the latest new form of attacking to be brought to Smash.
The method for using these Extra Skills comes straight from the Inkling's ability to refresh his ink supply by hitting Special+Shield and holding it down to fill up the gauge. For the most part these moves take inspiration from the Character Traits from Injustice. In Injustice hitting a certain button would unleash a special skill by that character. These Extra Skills could allow characters like Zelda and Sheik to transform into each other, have Paper Mario give himself a status buffing move like All or Nothing, whole new attacks like with Bowser's Fire Shot or Pac-Man's Namco Museum, or other wild ideas.
Most of the time, this new form of attack cannot be used in rapid succession. Most have a cool down that must be respected before you can use it again but a few have their own unique rules as to when and how they can be used. In Smashtopia, Zantok calls this new move the Extra Skill. Anyone can include an Extra Skill field for their characters if they desire, but it is an optional field.