The attack consists of Lucario concentrating Aura energy into a spherical shape in its paws, and then releasing it. It can be charged, and while doing so any other fighter who touches the sphere takes a small amount of damage.
Like an auto-charge special of many, the charge always starts automatically as soon as the B button is tapped and any attack button will fire the sphere during the charging period. However, Lucario can also continue to hold the sphere after it is fully charged, allowing it to take advantage of its damage dealing ability, or store it for later use, which is done by either moving to the left/right to roll or by pressing the shield button to shield cancel it, enabling Lucario to cancel the charging period at any given time. Cancelling it in the air will cause Lucario to cancel the charging period with an air dodge instead.
The sphere charges relatively fast, taking about half the time to fully charge as Samus' Charge Shot would, which equates to about 1.5 seconds. It can be used as a stage controller, powerful edgeguarding option, and a spammable projectile with uncharged Aura Spheres. The Aura Sphere hitbox while charging is a great way to refresh Lucario's moveset. It is also a high shield-pressure method and will quickly break an opponent's shield. The reason that it quickly breaks shields is the amazing speed behind the multiple hits of the charge which quickly wears away at the opponent's shield.
When fully charged, Aura Sphere travels in a straight line, but when fired without charging, it flies in an up-and-down wave motion. Due to Lucario's rather floaty aerial maneuverability, it is possible to constantly fire Aura Spheres while approaching foes from the air. A fully charged Aura Sphere's damage output changes dramatically as Lucario takes damage, starting out as low as 13% damage and up to 39% damage, but it all depends on the amount of damage Lucario has taken and stock advantage/disadvantage before attacking with it.
In addition, Aura Sphere's size, when fully charged, is quite different at higher percentages, possibly amounting to be three times as large when Lucario has taken heavy damage as opposed to when it is at 0%. Additionally, Lucario will still be charging the attack even at full charge, so the charging Aura Sphere hitbox can still be of use before being fired, or shield/roll cancelled.
Reverse Aura Sphere
It is known that the direction of every character's standard special move can be reversed by tapping the control stick/d-pad in the opposite direction immediately before or after pressing the special button (the exception being Sonic and his Homing Attack).
Lucario and its respective neutral special (Aura Sphere) stand out from others in regard to this technique, however. When performed with a directional jump, Lucario will not only turn around but also, with correct timing, its jump animation will reverse itself entirely (essentially bringing Lucario back to the origin of the jump.)
Coupled with the fact that Lucario's Aura Sphere generates a hitbox while charging, can be stored, and (if not fully charged) is a non-obligatory attack on on ground and in midair (charging process can be cancelled with a guard, as opposed to Link or Marth, whose charged neutral special moves are carried out as soon as the special button is released), Lucario's reversed Aura Sphere can be incorporated into gameplay beyond something as simple as a change of direction. Thus, the technique is notably named the Reverse Aura Sphere. This video demonstrates this technique.
Aura Sphere was significantly buffed in Smash 4 due to the improvements in Lucario's aura mechanic. It functions exactly like its Brawl incarnation, but the sphere can now achieve massive sizes, to the point that it completely covers Lucario's hurtbox by a great amount when at 190%, and completely oversizing Samus' fully charged Charge Shot. It deals less damage - 11% at 0% aura and 29% at 190% aura - but has increased knockback at high percentages, being a much stronger projectile than Brawl's through greater scaling at the cost of early base knockback and power. It also gains the ability to push Lucario back slightly if fully charged and reversed, giving Lucario an option to trick opponents.
Aesthetically, the sphere does not look similar to Shadow Ball, instead of being a smoky and slightly spiky energy-based ball with a spark-like core. As Lucario's aura increases, the sphere becomes more visually intense, having a massive wind shockwave charge effect when at 190%, and it has more pronounced blue aura "flames" when the sphere is shot, as well as different sounds. Lucario also has a more pronounced firing pose upon unleashing the attack, where it crouch-leans more forward and tilts a bit to its left while holding its arms more apart from each other.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Aura Sphere is a powerful special Fighting-type attack introduced in Generation IV, that deals 80 base power and has a property of not being affected by calculations of accuracy and evasion. It is an uncommon move, only available to a few evolutionary families of which several are Legendary Pokémon by level up.
Lucario learns this attack by leveling up in all games since its introduction. The move is often associated with Lucario, and is featured prominently in the movie Lucario and the Mystery of Mew as the prime example of Lucario's ability to manipulate Aura.