Genjutsu: Sharingan

Sasuke and Itachi Uchiha have both been shown activating their Mangekyō Sharingan to cast genjutsu despite already having their regular Sharingan activated, implying that this technique is enhanced in some way by the evolved dōjutsu, or that there is a similar technique utilised only by the Mangekyō Sharingan.
** Suigetsu Hōzuki commented that Sasuke's eyes had become even more convenient (さらに便利, sarani benri) after Sasuke cast genjutsu on Kiyoi Yotsuki with his Mangekyō Sharingan.
** As noted by Shisui Uchiha, Usually, the dōjutsu of the Sharingan requires a certain degree of intersecting gaze and the direct act of pouring chakra towards the target. However, Mangekyō Sharingan can eliminate that process. Simply by looking at the opponent's eyes, allows chakra to be poured even without eye contact. Moreover, the amount of chakra poured is several times greater than with the regular Sharingan. Consequently, the target falls into the dōjutsu in an instant, unaware of what has happened. (「通常の写輪眼の瞳術は、対象者に対してある程度の視線の交錯とチャクラを流すという直接的な行為が必要だ。だが万華鏡写輪眼は、その過程を排除することが可能だ。こちらが相手の眼を見るだけで、視線が交錯していなくてもチャクラを流しこむことができる。しかもその量は写輪眼の時よりも何倍も多い。そのため対象者は一瞬のうちに瞳術に墜ち、なにが起こったのかも解らない」, viz=“For the visual jutsu of a normal sharingan, your gaze has to intersect with the gaze of your intended target to some degree. You need that direct action, and the flow of chakra. But the Mangekyo sharingan allows me to sidestep all that a bit. I just have to look at my target’s eyes, and I can pour my chakra in, even if our gazes aren’t intersecting. And the amount of chakra is several times more than with normal sharingan. So the target falls into the visual jutsu instantly. They have no idea anything’s even happened.”). This allows for the genjutsu to be cast much more quickly, within less than a thousandth of a second, faster than the normal Sharingan can reliably bind a target under genjutsu, which can do so instead at about a a few hundredths of a second (数百分の一秒あまり, viz=less than a hundredth of a second). It appears this also allows for genjutsu to be cast without eye contact through obstructions, as Itachi was capable of placing insects within genjutsu through the body of their Aburame host.

A shadowy version of Sasuke's humanoid Susanoo first appears in a genjutsu he creates,

By making eye contact with a jinchūriki, Obito could use this technique to control the tailed beast sealed inside them, while its host remained unaffected. Sasuke displayed a similar ability, suppressing a portion of the Nine-Tails within Naruto Uzumaki.

Despite genjutsu working by altering the chakra flow of a target's cerebral nervous system, this technique affected Mecha-Naruto who does not have a brain. However, this wasn't required in Sasuke's usage.