Shadow Clone Technique

The Shadow Clone Technique is a clone technique created by Tobirama Senju.

Overview
This jutsu allows the user to create one or more copies of themselves. The user's chakra is evenly divided between themselves and their clones: creating one clone will give it half the user's chakra, creating two clones will give each a third of the user's chakra, and so on. Depending on how much chakra the user has and how many clones they make, this rapid depletion of their reserves can be dangerous. Because of this, usually only those of at least jōnin-level can safely use the standard Shadow Clone Technique. The Multiple Shadow Clone Technique – which creates hundreds of clones to the standard version's dozens – is unsafe to the point of being forbidden. Naruto Uzumaki is an exception to this rule; because he has access to the chakra of the Nine-Tails, he can create hundreds of clones without worry. Unlike the basic Clone Technique, shadow clones are physically real. They possess the same clothing, damage, and transformations as the user at the time of their creation. Most tools and weaponry are copied when the shadow clones are created, but certain chakra constructs and complicated technology won't be duplicated. Because they are visually identical and possess the same chakra as the user, shadow clones are indistinguishable from their original, not even by dōjutsu; Madara Uchiha can identify shadow clones using his Rinnegan.

Once created, shadow clones can serve a number of purposes. They can be used as decoys, either by keeping opponents preoccupied or, because they can't be told apart, concealing the original's identity. If enough shadow clones are created, they can simply overwhelm an opponent, pinning them down or attacking from multiple directions. Shadow clones can also be used simply to assist the user, joining the user for tactics they can't do alone or being sent to carry out tasks while the user is busy. Shadow clones possess all the same memories as the user, and so will react to unforeseen obstacles just as the user would in the same circumstances. Because they have their own chakra, shadow clones can perform other jutsu, including making more shadow clones.

Shadow clones usually disperse once they are struck hard enough, but they have been shown capable of enduring injuries for a time. When a clone's task is completed, it can disperse itself or be dispersed by its user. Once dispersed, the clone's experiences and remaining chakra are transferred back to the user. The shared experiences make shadow clones useful for intelligence gathering: a shadow clone can be sent into hostile territory, gather information without endangering the user, and disperse when finished, thus transferring what they've learned. Alternatively, shadow clones can recon an area, dispersing when they've found what they're looking for in order to let the user know the target's location.

Kakashi Hatake discovered that shadow clones can also accelerate training periods. For example, by training alongside one shadow clone for one hour, the user will have two hours' worth of training experience once the shadow clone disperses. By using hundreds of clones, training that would usually take weeks or months can be completed within a few hours. But because of how much chakra it takes to create just one shadow clone, and because the training itself will likely be arduous even without shadow clones, only Naruto can train in this way. Naruto is indeed able to make effective use of this shortcut, but the combined experiences come with combined mental fatigue, requiring that he take regular breaks.

Although they are separate, shadow clones maintain small links between themselves and their original, intended to allow the user to reestablish control when needed. Users can exploit this link to network their clones, causing changes in their own chakra to extend to all the others. In certain cases, this link can work against the user, such as when pain caused by the Nine-Tails' chakra was felt by all of Naruto's shadow clones. While a user and/or their clones are performing tasks that require concentration, they're unable to have more than a few shadow clones active at a time. In the novelisation of Boruto: Naruto the Movie, Naruto is consciously aware of what all his shadow clones around Konohagakure are doing. Although this allows him to have multiple conversations simultaneously, he has difficulty focusing on any one thing and the original Naruto becomes increasingly strained the longer the shadow clones are active; he drinks coffee and energy supplements in order to remain alert, but as soon as his concentration falters his shadow clones will all disperse.

Trivia

 * The hand-seal for the Shadow Clone technique is similar to the one predominantly-used by the Monkey King, Sun Wukong (孫悟空), the leading main character in the late 70's Japanese live-action TV series, Saiyūki, based off-of the 16th century Chinese classical novel, Journey to the West, by Wu Cheng'en; this is appropriate, as Naruto was based-off-of this novel, and one of Sun Wukong's favoured tactics is to make replications of himself against enemies in battle, etc.
 * Madara Uchiha used a shadow clone to fake his death. The clone apparently lasted indefinitely since the ruse was never discovered.
 * Sasuke Uchiha views Naruto's extensive use of shadow clones as a symbolic way to avoid loneliness.
 * In Naruto episode 129, Sasuke claims that the Sharingan can distinguish clones from the original. This contradicts the manga and other anime episodes.
 * In the anime, an ostrich named Condor appeared to use this technique, but in reality, it was just other ostriches.
 * In Kakashi Hiden, while waiting for his new arm, Naruto creates shadow clones with a one-handed seal: a cross with his left hand's index and middle finger.
 * In the anime, the Genjutsu World's Hinata Hyūga can make shadow clones.