Happy Medium
XXXNation
We don’t know much about genjutsu, because none of the protagonists use it and so it doesn’t get proper explanations the way ninjutsu does.
What we do know is that genjutsu works by somehow inducing a controlled modification in your target’s chakra flow, which somehow alters their perception of the world. This is why the easiest way to rid oneself of most genjutsu is to interrupt and reboot one’s chakra flow, much like you might switch a computer off and on again when it hits a snag: the rebooted chakra flow is not affected by the genjutsu.
(Lots of people, including most fanfic writers I’ve seen touch on the topic, seem to believe that a genjutsu is some kind of chakra construct one builds around oneself and the environment. That’s a Henge — a Transformation jutsu — one of the basic techniques taught at the Academy. Genjutsu has a completely different mechanism and potentially affects all the target’s senses.)
What we don’t know is how this works in practice. It appears as though a combination of specific sensory input combined with chakra can trap the person it hits in a genjutsu, going by Tayuya’s technique; it also appears as though genjutsu can be transmitted through touch, going by the mist-based genjutsu used by the Second and Fourth Mizukage as well as the technique used to interrogate Rin in Kakashi’s backstory. Generally, the principle seems to be that you send a bit of your chakra into your opponent and make it mess with his or her native chakra flow, and somehow sensory input can help with that.
However, we then run into the issue that the Sharingan doesn’t seem to have any mechanism to do that through simple eye contact, and, as the question mentions, Itachi managed to cast a genjutsu simply by pointing his finger at Naruto.
My personal interpretation of the latter case, and of the role of sensory input in general, is that the genjutsu user sends chakra into his or her victim, then uses sensory manipulation to trigger or control that chakra’s actions in some fashion. That would explain Tayuya’s technique (her flute’s sound carries chakra and manipulates it at the same time) and Itachi’s (he sent some chakra into Naruto at some point prior, then triggered the genjutsu with his gesture).
That doesn’t account for the sharingan’s power, though, and I don’t think I’ve seen anything that does.
What we do know is that genjutsu works by somehow inducing a controlled modification in your target’s chakra flow, which somehow alters their perception of the world. This is why the easiest way to rid oneself of most genjutsu is to interrupt and reboot one’s chakra flow, much like you might switch a computer off and on again when it hits a snag: the rebooted chakra flow is not affected by the genjutsu.
(Lots of people, including most fanfic writers I’ve seen touch on the topic, seem to believe that a genjutsu is some kind of chakra construct one builds around oneself and the environment. That’s a Henge — a Transformation jutsu — one of the basic techniques taught at the Academy. Genjutsu has a completely different mechanism and potentially affects all the target’s senses.)
What we don’t know is how this works in practice. It appears as though a combination of specific sensory input combined with chakra can trap the person it hits in a genjutsu, going by Tayuya’s technique; it also appears as though genjutsu can be transmitted through touch, going by the mist-based genjutsu used by the Second and Fourth Mizukage as well as the technique used to interrogate Rin in Kakashi’s backstory. Generally, the principle seems to be that you send a bit of your chakra into your opponent and make it mess with his or her native chakra flow, and somehow sensory input can help with that.
However, we then run into the issue that the Sharingan doesn’t seem to have any mechanism to do that through simple eye contact, and, as the question mentions, Itachi managed to cast a genjutsu simply by pointing his finger at Naruto.
My personal interpretation of the latter case, and of the role of sensory input in general, is that the genjutsu user sends chakra into his or her victim, then uses sensory manipulation to trigger or control that chakra’s actions in some fashion. That would explain Tayuya’s technique (her flute’s sound carries chakra and manipulates it at the same time) and Itachi’s (he sent some chakra into Naruto at some point prior, then triggered the genjutsu with his gesture).
That doesn’t account for the sharingan’s power, though, and I don’t think I’ve seen anything that does.
