Rasengan

The Rasengan is a powerful A-Rank technique invented by the Fourth Hokage, Minato Namikaze. After spending three years of developing it, however, he never saw the Rasengan completed, having never achieved the desired results before he died.

Minato's idea for the Rasengan was to take the Shape Transformation of his chakra to the "highest possible point" and then combine it with his chakra's nature affinity, creating a jutsu that did not need the aid of time-consuming hand seals to control. He died before he could accomplish this, but on shape manipulation alone, the Rasengan is still a very useful, powerful, and difficult jutsu 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 emphasizes 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 emphasizes 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, emphasizing stability of the sphere shape.

Although most users learn how to form the Rasengan with one hand, Naruto and Konohamaru Sarutobi use one or more shadow clones to help in its 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.

Konohamaru Sarutobi's Version
Konohamaru's version of the Rasengan is very small and requires a Clone to help keep it under control, holding it the same way Naruto uses the Odama Rasengan. It is about the size of a golf-ball, while the standard is the size of a rubber ball that can fit in a person's hand. The anime however made it the standard size, but still required the use of a clone to keep the control.

Jiraiya's, Naruto Uzumaki's, and Kakashi Hatake's Versions
Minato had taught both his sensei Jiraiya and his student Kakashi how to use the Rasengan while he was still alive, and after he had perfected it. Jiraiya later taught Naruto how to do the Rasengan during the search for Tsunade. The size of their Rasengan can fit in the palms of their hands, as large as a rubber ball. While Kakashi and Jiraiya can form and create it with one hand, Naruto's chakra control was so poor he required a clone to use it. Even after his chakra control had improved, he continues to use clones since it requires less training.

Minato Namikaze's Version
As the creator of the Rasengan, Minato had high control over the technique and could create it at an incredibly fast rate as shown during his battle with Madara Uchiha. Due to his high mastery of the technique, he could make his Rasengan larger than the standard yet still keeping it the same attack, which had the power near that of his son's Odama Rasengan, and inflicted so much damage to Madara's body, his arm melted off.

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 its counterpart, the Chidori. The compacted nature and moving speed of the chakra allows the Rasengan to grind into whatever it comes in contact with, inflicting major damage. This also tends to launch the target backward once hit. In an anime filler arc, Naruto was able to perform the Rasengan as a mid-range projectile technique by somehow throwing it into a wall.

The Rasengan was designed to use the caster's elemental affinities. Minato was unable to add his affinities to his Rasengan before he died. Kakashi Hatake tried to add his lightning element to his Rasengan, but was unable to and explains that he uses the Chidori and Lightning Cutter instead, which require a certain point of shape manipulation 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 Wind Release: Rasenshuriken.

Improvement
While in training, Naruto improved the Rasengan with the Great Ball Rasengan, after Jiraiya pointed out during their time away from Konohagakure 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.

Naruto can also wield a Rasengan in both of his hands while in Sage Mode. Sage Art: Great Ball Rasengan uses two Great Ball Rasengan, while Sage Technique: Rasenrengan uses two regular Rasengan. Jiraiya is suggested to have been capable of doing something similar; when teaching Naruto the Rasengan, he holds the Rasengan in one hand and does the water balloon exercise in his other hand.

Nine-Tails Influence
Naruto has shown that when he is under the influence of the Nine-Tailed Demon Fox's chakra, he can improve his use of the standard Rasengan and simultaneously make up for his shortcomings in performing the jutsu. The chakra cloak surrounding him in his one-tailed form serves as the shell to stabilize the Rasengan's chakra, thus he does not need a clone to help as usual. Due to the influence exhibited by the Nine-Tails' chakra, this Rasengan changes from its yellow or blue color to a purple, orange, or red color, made stronger than its standard version. Despite the fact that some video games give it its own name,, it is fundamentally the same jutsu.

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. The second Shippūden film is the only film to feature two new forms.