So I realize if you're not a Bleach watcher it probably won't be very funny. The "bankai" is what a more powerful Soul Reaper can do to release his zanpakuto to its final, most powerful form. They do it only when the S is really hitting the F.
There is actually quite a lot of humor in Bleach, but you have to pay attention. Most of it is character-based, so it only works once you know the characters.
*The world "shinigami" is the same word that's translated in Death Note to "death god," but in Bleach it's translated as "soul reaper."
P.S. I think Yamamoto would make an awesome action figure. And the variations! There would be "Old Man Yammie" before he lost his left arm, after he lost is left arm, and with his zanpakuto disguised as a walking stick or released into its flaming blade form. Nice.