Kamui

Kamui is a powerful dōjutsu which originates from the Mangekyō Sharingan of Obito Uchiha. A unique and specialised form of space–time ninjutsu, it allows the user to achieve two very distinct, yet closely associated feats – teleportation and "intangibility".

Overview
Kamui allows the user to transfer anything to another dimension. Once a target is sent to this dimension it is unable to escape. This technique is also able to make the user "intangible" by transporting portions of his body to the same pocket dimension. Obito was able to use this technique the moment he awakened his Mangekyō Sharingan, while Kakashi, who awakened his Mangekyō Sharingan at the same moment as Obito, didn't have access to it until sometime after Sasuke Uchiha's defection and made first use of it in the series during his first encounter with Deidara.

Kakashi Hatake likens the characteristics of the technique to those of Minato Namikaze's Flying Thunder God Technique, but notes that it seems to be much more versatile since it requires neither a seal nor a summoning tattoo to be performed. The side of the eye using the technique determines the specific property of it.

Teleportation
Each eye uses Kamui to teleport to the other dimension, though how this is done depends on the eye. By creating a swirling, distortion centered on the eye, the user is able to open a void into the other dimension. This technique's effect acts as an attractive force, both visibly drawing the target inwards and distorting their form until they disappear, although physical contact is required to absorb a target. After absorbing targets through this process, Obito or Kakashi is able to eject them at any time of their choosing with varying degrees of force. In doing so, Obito is able to attack using weapons of gigantic proportions and/or quantities with relative ease and at a moment's notice, giving opponents little opportunity to react accordingly.

Another unique trait of Kamui is that it allows the user to teleport themselves to any location they desire, including the other dimension. When using Kamui to teleport themselves, whatever the user is touching will be warped along with them and the user's chakra becomes untraceable. By his own admission, Obito is capable of finding anyone and teleporting to their location regardless of where they are. Despite not always being present, Obito can keep his targets under genjutsu while they are confined inside this pocket dimension and thus, keep them incapacitated for extended durations.

Left Eye
The left eye's primary use of Kamui is based on line of sight and, as such, does not require physical contact to warp a target into the other dimension. By focusing to create a barrier space and concentrating on the target within, Kamui will distort space and the target will be drawn into the other dimension. When Kakashi first used this technique, it required great concentration and was difficult to aim, dragging in only Deidara's arm when he aimed for his head. By the Invasion of Pain, Kakashi was able to use Kamui to stop a speeding nail from penetrating his head and later, during the Fourth Shinobi World War, could quickly warp a target as large as Gyūki, the Eight-Tails. He was also able to use Kamui to teleport out of the other dimension as well as eject Gyūki from it.

Right Eye
The unique trait of this eye is the ability of intangibility. When activated, any part of Obito's body that overlaps with a solid object is seamlessly warped to the other dimension, making it appear as though he is phasing through them. Using this ability, Obito can dodge most attacks, and by overlapping his entire body into large objects, he can not only conceal himself from view, but erase all traces of his chakra. Obito is also able to enter and travel through the ground using this ability, a characteristic he often exploits to ambush his opponents from below. Though he is unable to establish any physical contact while being intangible, Obito can still use chakra chains. He also seemingly possesses the capability to extend this intangibility to whatever he's touching at the moment of activation, as long as he is able to maintain some form of contact with at least a part of the desired item or person.

Drawbacks
Since this technique is shared by two different users of the same pair of eyes, both parties can use their own technique to counter the effectiveness of the other, thus making it useless to use against one another directly. Obito can extend this negation to targets outside of his own body, as he negated Kakashi's attempt to decapitate the Demonic Statue of the Outer Path's head. Similarly, Kakashi was able to escape Obito's attempt to trap him in the other dimension.

Left Eye
The Mangekyō Sharingan is a dōjutsu that, with continuous usage, burdens the user with the loss of their eyesight. Since this is the source of Kamui, using it in rapid succession puts Kakashi's body at risk as well. As his Sharingan was transplanted, usage of this technique poses an even greater risk to Kakashi. Similar to Amaterasu, repeated use of the technique puts a great deal of strain on the user, causing his left eye to bleed if overused.

Kakashi was originally only able to use this technique up to at least three times in a given day, even when starting at full strength. At first Kakashi's aim and control over the size of the barrier was imperfect. Over time his aim and control improved, as he was able to accurately warp away small and fast moving objects, such as a nail. Kakashi is even able to consecutively warp away two fast-moving arrows. Later, he became able to use this technique several more times in a single day.

Right Eye
In Obito's case, the primary weakness of this technique is that both aspects cannot be used concurrently, as he must become and remain tangible in order to transport himself or others, providing a small opportunity in which he is unable to pass through objects and thus, is vulnerable to injury. This deficiency means that he must also materialise when ejecting stored items as well, presenting the same vulnerability. According to Konan, Obito can only maintain his intangibility for approximately five consecutive minutes at a time, after which he is presumably rendered completely solid for a duration. She also reveals that the time it takes him to absorb something is proportional to its mass, and that it takes longer to absorb himself than another person or object.

Additionally, due to the nature of Obito's intangibility, the sections of his body residing in the other dimension can still interact normally with other objects that are also contained within this dimension, meaning they can be damaged by such items. As this technique originates from Obito Uchiha's Sharingan, of which both Kakashi and Obito possess one of, the two share the same dimension when teleporting objects, a characteristic that Kakashi exploited to injure Obito on multiple occasions during the Fourth Shinobi World War.

Unlike Kakashi or any Mangekyō Sharingan users, Obito's extensively continuous usage of the Kamui technique does not seem to tire him at the very least, nor even cause his eyesight to deteriorate.

Influence
Kamui is the word for a divine being in Ainu mythology, although it is almost never written with kanji in modern times. In a more general Japanese context, Kamui refers to the might and majesty of the gods, particularly the Shintō Kami.

Trivia

 * The Third Databook seems to hint that other Mangekyō Sharingan users can access the technique through sheer training of the eye, though this is unconfirmed.