Rasengan

The Rasengan is a powerful A-rank technique invented by the Fourth Hokage, which he created by observing the Tailed Beast Ball. Minato spent three years developing the Rasengan; his plan was to take the shape transformation of one's chakra to the "highest possible point" and then combine it with his chakra's nature affinity, creating a technique that did not need the aid of time-consuming hand seals to control. He died before he could accomplish this, but on shape transformation alone, the Rasengan is still a very useful, powerful, versatile, and difficult technique to learn and master.

Usage
In Part I, Jiraiya taught Naruto Uzumaki how to use the Rasengan, breaking the learning process into three steps:
 * The first step was learning how to burst a water balloon by spinning the water inside in multiple directions at once using only the user's chakra, which emphasises rotation.
 * The second step was for the user to burst a rubber ball, and since rubber is thicker, and air is completely different from water, it forces the user to use denser chakra to break it. This step emphasises power.
 * The third and final step was to combine both step one and two by spinning chakra inside a balloon without moving the balloon itself, emphasising stability of the sphere shape.

Although users generally learn how to form the Rasengan with one hand, Naruto and Konohamaru Sarutobi use one or more shadow clones to help in the creation. By having the hands of the shadow clones act as shells, Naruto and Konohamaru can complete and use the technique without using any extra time to master their chakra control.

Effects
The Rasengan does not require any hand seals to use, relying only on the chakra control of the user. Once it is complete, it becomes self-sustaining, which does not give the Rasengan a definite limit like the Chidori. The compacted nature and moving speed of the chakra allows the Rasengan to grind into whatever it comes in contact with to inflict major damage. This tends to launch the target backward after being hit or, if the target is lying on the ground or being launched to the ground, the destructive force is enough to form a crater. In an anime filler arc, Naruto was able to somehow throw the Rasengan into a wall, making it a mid-range projectile technique. It has been noted a few times that the Rasengan is superior to its counterpart, the Chidori, by Jiraiya, Kakashi Hatake, and even Sasuke Uchiha after seeing the after effects of the Rasengan on a water tower.

The Rasengan was designed to use the caster's elemental affinities. Minato was unable to add his affinities to his Rasengan before he died, while Kakashi Hatake tried to add his lightning element to his Rasengan, but was unable to and explains that he created the Chidori and Lightning Cutter instead, which require a certain point of shape and nature manipulation itself. Naruto was the first to add his wind affinity to his Rasengan to create the Wind Release: Rasengan, and even advanced it up to the point that he could create the far more powerful and destructive version, Wind Release: Rasenshuriken.

Improvement
While Naruto has found new ways to use this technique, he still needs clones to make his Rasengan.

During his training with Jiraiya, Naruto improved the Rasengan with the Big Ball Rasengan. An anime exclusive flashback explained that the Big Ball Rasengan's invention resulted from Jiraiya's remark that it would be easier to hit an opponent with a larger Rasengan. Later, Naruto mixed his wind-natured chakra to start developing the Wind Release: Rasengan, as it was originally intended. Naruto took the Rasengan even further than his father by creating the Wind Release: Rasenshuriken, a variant of the Wind Release: Rasengan. However, this new technique not only damaged the opponent's chakra circulatory system, but also inflicted the same type of damage to Naruto's arm, making it a double-edged sword. Even so, Naruto overcame this obstacle as shown during his fight with Pain when, in Sage Mode, he threw the Rasenshuriken, preventing any damage done to his own body, and increasing its power, causing disintegration on contact.

Naruto can also wield a Rasengan in both of his hands while in Sage Mode. Sage Art: Big Ball Rasengan uses two Big Ball Rasengan, each the size of his own body, while Sage Technique: Spiralling Serial Spheres uses two regular Rasengan. Jiraiya is suggested to have been capable of doing something similar. When teaching Naruto the Rasengan, he held the Rasengan in one hand and does the water balloon exercise in his other hand. Both Jiraiya and Naruto can create even larger Rasengan, the Ultra-Big Ball Rasengan and Sage Art: Ultra-Big Ball Rasengan respectively. Naruto can even mass a large group of Ultra-Big Ball Rasengan and launch them at one target as the Sage Art: Many Ultra-Big Ball Spiralling Serial Spheres, which could even damage the Nine-Tailed Demon Fox.

Naruto is now using his chakra arms in his Nine-Tails Chakra Mode to create a new Rasengan, the Tailed Beast Rasengan, which is even more similar to the Tailed Beast Ball than the Rasengan originally was.

Nine-Tails Influence
Naruto had shown that when he was under the influence of the Nine-Tails' chakra, he could improve his use of the standard Rasengan and simultaneously make up for his shortcomings in performing the technique. The chakra shroud surrounding him in his one-tailed form served as the shell to stabilise the Rasengan's chakra, thus he did not need a clone to help as usual. Due to the influence exhibited by the Nine-Tails' chakra, this Rasengan changed from its yellow or blue colour to a purple, orange, or red colour, made stronger than its standard version. Despite the fact that some video games give it its own name, and, it is fundamentally the same technique.

During his training with Killer B, Naruto was informed that the Rasengan follows the same properties as the Tailed Beast Ball, in that it's a technique that compresses chakra into a ball. The main difference between the two is the rotation used to stabilise the Rasengan. Because the Tailed Beast Ball is unusable unless Naruto can completely transform into the Nine-Tails, Naruto must instead learn how to form a Rasengan by mixing a balance of black "positive" and white "negative" chakra at an 8:2 ratio. He can form the Tailed Beast Rasengan, but it isn't yet fully functional. Naruto has also created new variants of the Rasengan with chakra arms while in Nine-Tails Chakra Mode such as the Spiralling Strife Spheres, the Spiralling Absorption Sphere, and the Planetary Rasengan. Using the chakra arms, he is also able to create a miniature version of the Rasenshuriken with just his index finger and two small chakra arms.

Trivia

 * Every Naruto film, except the third Shippūden film, featured a new version of Rasengan. Here, Naruto forms a standard Rasengan without the aid of a Shadow Clone by instead using both hands to form it, and then infuses it with a certain element from the film's central plot, and uses the attack to strike down the main antagonist. Naruto Shippūden 2: Bonds is the only film to feature two new forms instead of one.
 * It was hinted by Killer B that the Rasengan is too perfect for Naruto to be a coincidence. He stated it's almost as if Minato Namikaze designed the Rasengan specially to pass on to Naruto.