When called forth the Bankai form of the Zanpaku- to raises the Soul Reaper's powers to the highest possible level.
Evidence of Evolution
When released a Bankai alters the wielder's powers much like the Shikai does, only at an even higher level than its predecessor. The Bankai also alters the form of the Zanpaku- to, but it also can alter the way the wielder appears. Ichigo and Renji both change in appearance and power whereas Byakuya's Zanpaku to changes but he doesn't.
How Does It Work?
Like the Shikai release the wielder utters their release command while connecting with their Zanpaku- to's spirit. The complete union of the two gives them their full power and opens up several new levels to their attacks too. The true form of the Zanpaktou and has immense power. For the most part, only Lieutenant level and higher Shinigami have obtained this level because of the immense time it takes to train to call out the true power of the sword. The powers, form, and limit, greatly depend on the soul, spirit energy, and personality of the wielder. But also depends on the sword itself. As seen with Ichigo Kurosaki, one must become one with their Bankai to finally release it. The shinigami must search deep in themselves and defeat the Bankai as a physical being to be able to use this power.