The Sharingan is a technique used by Madara Uchiha, Sasuke Uchiha, and after recieving Sasuke's eye, Xirbealg.
The Sharingan is only first unlocked under some stressful or emotional condition, after which, though, the Sharingan may be used freely. When the Sharingan is first activated by a user, it has a different number of tomoe around the central pupil, varying between users. The wielder of the Sharingan, through continued use and training, evolves it further which involves gaining more tomoe, up to a maximum of three. As the Sharingan evolves, the user gains heightened abilities. The only level up from the three tomoe Sharingan is the Mangekyo Sharingan, which is gained by the emotional stress stemming from the loss of someone close to the user.
The first of the Sharingan's powers is being able to see an enemy's energy flow.
The Sharingan's second-most prominent ability grants the user an incredible clarity of perception, allowing them to easily recognize different forms of energy. This also allows the user to pick up on subtle details, enabling them to read lip movements or mimic something like pencil movements. As the Sharingan evolves, gaining more tomoe seals, this ability extends to being able to track fast-moving objects before finally giving some amount of predictive capabilities to the user. However, even though the user may be able to see an attack or know it is coming, their body may not have time to react.
The Sharingan's third and most well-known ability is that it grants the user the ability to memorize any technique that he or she witnesses with perfect accuracy, allowing the user to use the techniques as his own, or even modify them to create his own new techniques. In order to reproduce a copied jutsu, however, one must have the necessary skill or ability to perform them.
The Sharingan's final ability is capable of inducing a unique brand of hypnosis that involves suggesting actions and thoughts to the opponent through simple eye contact. When used in conjunction with the other aspects of the Sharingan, this allows the user to perfectly mimic every move their opponents make even before they do, making it appear as if the user can "see the future". More advanced Sharingan users can take the hypnosis ability even further by tricking or even trapping their opponent within an illusion. The potency of the Sharingan's illusion is dependent on the user's knowledge and skill with implementing illusions.
Although the Sharingan consumes the user's energy while active, the power levels that the user must exert is so insignificant that they can keep it active constantly. However, when a Sharingan is transplanted to a non-Uchiha, the chakra drain is significant, leading them to cover it up when not in use, to stop the drain.
Receiving the Sharingan
As already mentioned, the Sharingan needs to be activated by the user himself or herself under a difficult, emotional stress. After its first appearance, the user can use the Sharingan any time he or she pleases. It is also possible to transplant Sharingan eyes from one person to another, as occurred with Xirbealg, who absorbed his left eye's Sharingan from his injured comrade, Sasuke Uchiha. Because of this, Xirbealg's Sharingan cannot be deactivated, and as using the Sharingan requires much more energy from him than it does from an Uchiha clan member, he usually covers it with his headband to conserve power. Also, Xirbealg needs to rest for a longer period of time after using his Sharingan extensively in battle.
Transplanting the Sharingan eyes from a living or deceased Uchiha is the only way to avoid the loss of sight over time from the repeated use of the Mangekyo Sharingan. The user is then said to have an "Eternal" Mangekyo Sharingan.