Rasengan

The Rasengan is a spinning ball of chakra formed and held in the palm of the user's hand, which is only known to a handful of shinobi.

Overview
The Rasengan was created by Yuusei Urahara.He intended the jutsu to be the highest form of shape transformation. The Rasengan does not require hand seals to perform. Once it is formed, it also does not require any additional chakra to sustain it, which means that there is no definite limit to how long the Rasengan can be maintained. However, the Rasengan will dissipate if it collides with something, even if it's an unintended target. Though the Rasengan's shape is consistent from user to user, its size varies from only barely large enough to be noticeable to much larger than the user's hand. With greater experience with the Rasengan comes a larger size.

The Rasengan grinds into its target, propels them along the Rasengan's trajectory - either away from the user or into the ground beneath them - and at some point bursts, causing severe damage at the point of contact.

Learning
The Rasengan requires extremely refined chakra control, something that very few ninja naturally possess. For this reason, the Rasengan is difficult to master. In order to help others gain the necessary chakra control, learning to use the Rasengan is broken into three steps:
 * 1) Rotation - the user learns to spin their chakra in multiple directions at once. To help in this regard, users may be given a water balloon so that they can mark their progress by the churning water; this step is complete once the users is able to burst the balloon with their chakra. It is important for the user to know which direction their body naturally spins its chakra for this step.
 * 2) Power - the user must increase the volume and density of chakra they output. To help in this regard, users may be given a rubber ball: there is no water inside to help them burst the exterior and the rubber shell is thicker than a balloon's.
 * 3) Containment - the user must combine the first two steps into a contained sphere. To help in this regard, users may be given a balloon to help them visualise the intended shape; if the balloon is popped or otherwise moves, mastery is not yet achieved.

Completion of the final step is by far the most difficult. For this reason, Miroku Oki uses Earth Clones to help him with forming the Rasengan, the clone's hands being used to create the shell. This gives each of them a shortcut to using the Rasengan without spending the time to fully master their chakra. He does eventually learn to use the Rasengan without clones.

Improvement
There are two broad methods by which the standard Rasengan's offensive potential can be increased: making the Rasengan itself bigger (as with the Big Ball Rasengan) or by using more than one Rasengan at a time (as with Spiralling Serial Spheres).

Senjutsu Influence
When in Sage Mode, the user can use a senjutsu-enhanced version known as Sage Technique: Rasengan. It is fundamentally the same as the normal version, except stronger. While in Sage Mode, users have an easier time forming larger Rasengan, such as the Ultra-Big Ball Rasengan.

Kekkai Genkai Influence
When merged with certain Kekkai Genkais, such as a unique Release, experienced users can create different kinds of Rasengan innately superior to a one-element Rasengan. For example, Tamura combined the Rasengan with his Explosion Release to create the powerful Rasenshuryudan.