Kirby Super Star has a few abilities that are used in speedruns. Most abilities in the game are disregarded in favor of abilities that prioritize speed and damage.
Jet
Jet is the most common ability in KSS speedruns, being used for quick movement and combat. It is used throughout the entirety of Dyna Blade, Milky Way Wishes Any%, and most of Revenge of Meta Knight. It can also be used in Great Cave Offensive Any% if opting for the no mix route. Jet is one of the best moves for horizontal movement, and is best applied when chained sequentially in the air. Jet is also great at moving up vertically by using its hover, which is faster than Kirby's standard hovering.
Holding Y while standing still will allow Kirby to charge his Jet. This charge can be released immediately or be stored by interrupting the charge through moves such as guarding or jumping. If Kirby presses B in the air while a full charge is stored, he will perform a rocket boost that propels him upward. This is typically used for skipping minibosses and miniboss quick kills.
Standing next to an enemy while holding left or right and pressing Y will cause Kirby to grab and shoot them away (officially called "Jet Blow" in-game). Performing this action with a stored full charge will shoot Kirby into the air ("Rocket Dive"). While there are niche applications in speedruns that use these moves, it is easy to unintentionally use them when moving with Jet, and doing so can waste a decent amount of time. They can be avoided by letting go of the D-pad while pressing Y near an enemy so the move isn't performed.
For more information on the Jet ability, see Jet.
Wheelie Rider
Wheelie Rider is an ability mainly used in Great Cave Offensive Any% and in some of Milky Way Wishes 100%. In GCO, Wheelie is used in tandem with Hammer.
Wheelie is spawned by creating a Helper from a Wheel ability. Wheelie Rider is better than base Wheel because of a faster top speed and having the ability to jump while moving. In addition, Kirby is able to carry a different ability with him while utilizing Wheelie.
Wheelie Rider has the fastest horizontal speed in the game, but is outclassed by Jet in many scenarios because of its lack of vertical speed. Wheelie Rider also requires a Helper, which consequentially gives bosses more health due to having a second player.
For more information on Wheelie Rider, see Wheel.
Plasma
Plasma is an ability used in some of Revenge of Meta Knight, most of Milky Way Wishes 100%, and The Arena. It can also be used in Milky Way Wishes Any% if opting for Plasma route.
Plasma is one of the strongest abilities for combat in KSS. It is charged by pressing multiple D-pad inputs in a row, so the faster someone is able to mash the D-pad, the faster Kirby charges Plasma up to full capacity. Plasma reaches full charge once the D-pad is pressed 16 times.
Mastering Plasma can be tough for runners because of the endurance needed to mash the D-pad throughout runs. If using Plasma for long periods, it is advised to stretch your wrists and/or hands, as well as to take breaks to avoid injury.
For Plasma mashing techniques, see Mashing Techniques. For more information on the Plasma ability, see Plasma.
Hammer
Hammer is an ability used throughout the entirety of Great Cave Offensive Any% and most of Milky Way Wishes Any%.
Hammer is another strong ability for combat in KSS, but is outclassed by Plasma due to its small hitboxes and long charge/cooldown time. Hammer's most utilized moves are Up + Y ("Hammer Flip") and Dash + A ("Hammer Throw"). Hammer Throw discards the Hammer ability, but can be re-obtained easily in Milky Way Wishes.
Hammer is primarily used in boss fights, as mini-bosses can be taken care of quickly through Jet or Wheelie Rider by using miniboss quick kills.
For more information on the Hammer ability, see Hammer
Wing
Wing is an ability used for a small period of time in Great Cave Offensive 100%, Milky Way Wishes 100%, and Gourmet Race. It can also be used in Great Cave Offensive Any% if opting for the no mix route.
Although outclassed by Jet, Wing has very fast aerial speed by using its "Hover Flap", which replaces Kirby's standard hover. Wing also allows for very fast vertical speed by repeatedly using down + Y ("Dive Bomb") and cancelling the move by pressing B before Kirby starts moving downward.
For more information on the Wing ability, see Wing.
Mix
Mixing is used in Spring Breeze, Dyna Blade, Great Cave Offensive Any% and 100%, and Revenge of Meta Knight.
Mixing is an important element in many Kirby speedruns, as it allows Kirby to obtain abilities that otherwise would not be obtainable in certain sections.
For more information on Kirby Super Star mixing, see KSS/Mix. For more general information on mixing and tricks to make it easier, see Mix.