Too lazy to go find the translation right now. Basically Aizen explains that his bankai allows him to instantly hypnotise anyone who sees it. At some point before the series started, Aizen brought all the captians and vice captians into a room to show them his bankai and from then on gave them the illusion that it was something completely different from its real abilities (like gravity manipulation or something). So he has basically made everyone see what he wants them to see, the catch being that it doesn't work on a blind man, so Tousen was on Aizen's side the entire time.