During the Arrancar war saga in Bleach, Yamamoto fought against Aizen before the arrival of Ichigo. In the end of the fight he used a kidou that almost killed himself tying to defeat Aizen. If I remember, this kidou uses the body as a "fuel" for it to work.
But in the new saga, he showed his bankai against Juha Bach. And it's very powerful.
He said that never used his full power, why didn't he use it against Aizen? If he underestimated him, why would he use that kidou that almost killed him and ended up losing the fight, instead of using his bankai?