In Bleach, Aizen's sword is claimed to be one of the most powerful, as it can control the human senses. As I remember, it was said that if you have seen it once, you've already lost to it. Can someone explain to me exactly what it does?
The special ability of Kyōka Suigetsu is Kanzen Saimin (完全催眠, Complete Hypnosis).
It controls the five senses to the point where it can make the target misinterpret another person's form, shape, mass, feel, and smell to be an enemy's. The initiation condition for the hypnosis is to show the enemy the release of Kyōka Suigetsu. After seeing it just once, this person will completely succumb to the hypnosis. Afterwards, every time it is released, this person will fall under the thrall of the hypnosis. The hypnosis is long lasting; the Visored are still affected by Kyōka Suigetsu even though they were hypnotized at least 110 years ago. Because the spell works when one see the ritual, those who cannot see are consequently immune to the hypnosis.
When Aizen turns off the hypnosis by force of will, the image shown melts away or shatters. The complete hypnosis of Kyōka Suigetsu is absolutely flawless; even if the target is aware of being under the hypnosis, they cannot resist its influence. The hypnotic abilities of Kyōka Suigetsu make it a highly useful tool in combat, for Aizen can create simple or complex illusions to deceive his opponents.
For example, by creating a decoy of himself, or other illusions to distract his opponents, Aizen can hide his own movements and attack with impunity. His opponent will remain focused on the illusion until it is too late to react.While the illusions Kyōka Suigetsu creates are quite powerful, those with considerable power and skill themselves may be able to note slight differences in the illusions compared to the real thing, even if they are not able to determine what exactly is different.
4th Division Captain Retsu Unohana detected something was amiss with Aizen's fake corpse, even though she did not realize what it was until seeing Aizen alive.The Reiatsu of Kyōka Suigetsu gives it away to those keen enough to sense its actual presence past the illusions created by its complete hypnosis.The sole way to escape the ability of Kyōka Suigetsu is to touch the blade itself before complete hypnosis is activated.
I haven't seen the entire series or read the manga, but from what he said before he fleed the Soul Society, it was apparently absolute hypnosis. Take this as an example:
When Ichigo attacked Aizen, he had already seen Aizen's sword, so Aizen's Zanpakuto could use the hypnosis "Aizen blocks Ichigo's attack instantly with one finger, and no one sees Aizen's counter attack"
As for how he could use hypnosis like that, maybe Aizen's Zanpakuto warped space (like how Touzen's Zanpakuto did when he was fighting Kenpachi) and created a space that robbed all senses, but because of the hypnosis, no one actually saw it, as Ohana even commented that Aizen previously showed everyone that his Zanpakuto was a water-based one, and she was shocked when Aizen revealed that he did it on purpose to fool everyone and to make them victims of his Zanpakuto.
First off, Aizen is a genius of nearly unmatched brilliance for most of his entire plan. One can say that he made some bad choices later in his plan concerning his treatment of underlings, the showdown with the Soul Society, and giving Ichigo a chance. However, that's more of an issue of character flaws than him not being a genius.
The ability that Aizen uses is not his Bankai, but rather his Shikai, all according to him of course. One falls under the hypnosis when one sees his sword as he releases its Shikai by speaking its name. The hypnosis apparently lasts forever after you have seen it once, given the fact that it lasted 100 years on several other Soul Reapers.