I wrote this up commenting on edenfalling's LJ, actually, but it didn't gather much discussion there and I wanted to throw it out for people to go over.
WARNING: All of the following is my own speculation, not established canon.
See, the way I figure the hypnotic aspect of the Sharingan works starts with my theory on the Byakugan (you'll remember that the two are known to be related). Which essentially says that it has absolutely nothing to do with vision, per se, but rather allows its owner to perceive chakra (thereby seeing the tenketsu and inner coils system), and emit small amounts of same to bounce off of their environment and return back. In short, the 'zoom' and 'area coverage' aspects of the Byakugan are basically chakra sonar, like a bat or dolphin.
The proto-Uchiha developed out of the Hyuuga when some clever light noticed that genjutsu...
oh. Oh, my. I just figured out what determines whether a jutsu is ninjutsu or genjutsu.
A ninjutsu, no matter what its ultimate effect, sends chakra out to manipulate the inanimate world around it. A genjutsu, again without regard to its ultimate effect, sends chakra out to manipulate another living chakra system.
...anyway. Casting a genjutsu doesn't take that much more energy than the Byakugan was dealing with anywho, it just means that it has to be handled differently. (*pikupiku* Another brainstorm - Mangekyou Sharingan as function which allows the Sharingan to expand its targets beyond other living things? Discuss.) Having a specialized organ capable of projecting genjutsu would greatly increase a shinobi's speed, subtlety, and overall effectiveness, since they can just use it, rather than having to contort their entire chakra system to create a genjutsu, the way doing it with seals requires, so there's an obvious motivation for developing it.
The human brain and optic nerves are not built to handle the amount of sensory information the Byakugan throws at its users - this suggests that either most of the problem learning to use the thing is filtering the data rather than actually expanding its function, or, the modifications made to the Hyuuga eye extend deeper than just the cornea. If the second, then that additional 'processing power' available to process sensory input, when applied to a normal field of view, would let an Uchiha examine everything around them in the kind of detail that would take most people hours to cover even a few square feet. (possible argument for this is the way Neji was able to examine Hinata closely enough to pick up her 'tells' he'd have spent years learning - IIRC, he didn't have his Byakugan on yet)
I've heard fanfics suggest that the jutsu learned through the Sharingan are 'stored' in it, and can't be accessed when it's inactive, but I can't recall any sort of canon support for that theory. If the data being processed never goes higher in the brain than the optic nerve, then it might indeed be that way, but I think it's more likely that they're just able to 'look' at what their opponent is doing so intensely that the second or so the jutsu takes is like hours of study. Naturally, being able to examine the world and pick up its fine details would also go a long way towards breaking genjutsu centered around the visual sense, since even the best genjutsu user would miss some kind of fine detail in their presentation.
The appearance of the perceptive function after the genjutsu bit had been engineered into their eyes might or might not have been deliberate, probably depending on which version of the 'learning' function you use.
Anyway, the point of all this is that I think that the Sharingan is just using an extraordinary mechanism to create a perfectly ordinary genjutsu.
Ja, -n
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===============================================
"V, did you do something foolish?"
"Yes, and it was glorious."
WARNING: All of the following is my own speculation, not established canon.
See, the way I figure the hypnotic aspect of the Sharingan works starts with my theory on the Byakugan (you'll remember that the two are known to be related). Which essentially says that it has absolutely nothing to do with vision, per se, but rather allows its owner to perceive chakra (thereby seeing the tenketsu and inner coils system), and emit small amounts of same to bounce off of their environment and return back. In short, the 'zoom' and 'area coverage' aspects of the Byakugan are basically chakra sonar, like a bat or dolphin.
The proto-Uchiha developed out of the Hyuuga when some clever light noticed that genjutsu...
oh. Oh, my. I just figured out what determines whether a jutsu is ninjutsu or genjutsu.
A ninjutsu, no matter what its ultimate effect, sends chakra out to manipulate the inanimate world around it. A genjutsu, again without regard to its ultimate effect, sends chakra out to manipulate another living chakra system.
...anyway. Casting a genjutsu doesn't take that much more energy than the Byakugan was dealing with anywho, it just means that it has to be handled differently. (*pikupiku* Another brainstorm - Mangekyou Sharingan as function which allows the Sharingan to expand its targets beyond other living things? Discuss.) Having a specialized organ capable of projecting genjutsu would greatly increase a shinobi's speed, subtlety, and overall effectiveness, since they can just use it, rather than having to contort their entire chakra system to create a genjutsu, the way doing it with seals requires, so there's an obvious motivation for developing it.
The human brain and optic nerves are not built to handle the amount of sensory information the Byakugan throws at its users - this suggests that either most of the problem learning to use the thing is filtering the data rather than actually expanding its function, or, the modifications made to the Hyuuga eye extend deeper than just the cornea. If the second, then that additional 'processing power' available to process sensory input, when applied to a normal field of view, would let an Uchiha examine everything around them in the kind of detail that would take most people hours to cover even a few square feet. (possible argument for this is the way Neji was able to examine Hinata closely enough to pick up her 'tells' he'd have spent years learning - IIRC, he didn't have his Byakugan on yet)
I've heard fanfics suggest that the jutsu learned through the Sharingan are 'stored' in it, and can't be accessed when it's inactive, but I can't recall any sort of canon support for that theory. If the data being processed never goes higher in the brain than the optic nerve, then it might indeed be that way, but I think it's more likely that they're just able to 'look' at what their opponent is doing so intensely that the second or so the jutsu takes is like hours of study. Naturally, being able to examine the world and pick up its fine details would also go a long way towards breaking genjutsu centered around the visual sense, since even the best genjutsu user would miss some kind of fine detail in their presentation.
The appearance of the perceptive function after the genjutsu bit had been engineered into their eyes might or might not have been deliberate, probably depending on which version of the 'learning' function you use.
Anyway, the point of all this is that I think that the Sharingan is just using an extraordinary mechanism to create a perfectly ordinary genjutsu.
Ja, -n
===========
===============================================
"V, did you do something foolish?"
"Yes, and it was glorious."