Kamui

Kamui is a powerful Mangekyō Sharingan dōjutsu that creates 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, when used through Obito's right eye, is also able to make the user "intangible" by transporting portions of their body to the same pocket dimension. Obito was able to activate Kamui the moment he awakened his Mangekyō Sharingan, while Kakashi, who awakened his Mangekyō Sharingan at the same moment as Obito without realising, didn't have access to it until some time after Sasuke Uchiha defected from Konohagakure.

Kakashi Hatake likens the characteristics of this technique to those of the 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 range and detailed properties.

Teleportation
Each eye is able to transport targets by creating a swirling distortion that opens a void into the other dimension. When both eyes are used together on the same target, the process works twice as fast. This technique's effect acts as an attractive force, both visibly drawing the target inwards and distorting their form until they disappear. A user of the right eye requires physical contact to absorb a target, while a user of the left can do so from a distance. After absorbing targets through this process, the user is able to eject them from the eye used at any time of their choosing, demonstrating the ability to eject objects with varying degrees of force. This is demonstrated by Obito, who ejects weapons of various proportions and/or quantity with relative ease and at a moment's notice, giving opponents little to no opportunity to react accordingly. While in Kamui's dimension, the user can transport targets out of said dimension at any time, even without making physical contact with them.

Another unique trait of Kamui is that it allows the user to teleport to virtually any location they desire, including the other dimension, by absorbing their own body into the swirling distortion. When using Kamui to teleport, whatever the user is touching will be warped along with them and their chakra becomes untraceable. Despite not always being present, Obito can keep his targets under genjutsu while they are confined inside this technique's pocket dimension, thus keeping them incapacitated for extended durations.

Long-range
An ability belonging to Obito's left eye, the long-range version of Kamui is primarily 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.

Short-range
The short-range version of Kamui, belonging to Obito's right eye, has a unique variation of teleportation akin to intangibility. When activated, any part of the user'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. This ability can dodge most attacks, and by overlapping his entire body into large objects, the user can not only conceal himself from view, but temporarily erase all traces of his chakra. This ability can also be used to travel through the ground; a characteristic that Obito often exploited to ambush his opponents from below. The user 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 could extend this negation to targets outside of his own body by concentrating on the warped space, as he negated Kakashi's attempt to decapitate the Demonic Statue of the Outer Path. Similarly, Kakashi was able to escape Obito's attempts to trap him in the other dimension and influence attacks to hit the former should he attempt to phase through them.

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 put Kakashi's body at risk as well. As his Sharingan was transplanted, usage of this technique posed an even greater risk to Kakashi. Similar to Amaterasu, repeated use of the technique puts a great deal of strain on the user, and caused Kakashi's left eye to bleed when 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, as well as consecutively warp away two fast-moving arrows. Later, he became able to use this technique several more times in a single day and attempted to use it to decapitate the Demonic Statue of the Outer Path, although he required a considerable amount of time to prepare the necessary amount of chakra and the technique was ultimately negated by Obito. After being given chakra by Kurama, Kakashi was able to use his long-range variant of Kamui to quickly transport a target as large as Gyūki to the other dimension and later eject the tailed beast from his eye, as well as utilise the technique more times than he would usually be capable of. He estimated that the influence of Kurama's chakra, when given directly by Kurama, made his usage of Kamui at least three times more powerful than usual.

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. In order to bypass this weakness and remain in Kamui's dimension for a longer duration, Obito has to "solidify" and absorb his entire body. She also revealed 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 said dimension, meaning they can be damaged by such items, a characteristic that Kakashi exploited to injure Obito on multiple occasions during the Fourth Shinobi World War. While controlling the Ten-Tails, Obito was unable to extend his intangibility to the beast or his means of connecting to its head, and was separated from it as a result. After becoming the Ten-Tails' jinchūriki, Obito noticed that he could no longer use Kamui's ability to let objects slip through him. Madara, who was able to teleport himself as the Ten-Tails' jinchūriki into the Kamui dimension, proved that this part of Kamui still works.

Unlike most other Mangekyō Sharingan users, Obito's continuous usage of Kamui does not seem to tire him or 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

 * In the sixth movie and several anime episodes, Obito is shown to be able to access Kamui without having his Mangekyō Sharingan activated.