As a Shinigami grows in power, he develops a greater bond to his Zanpakuto, and can unlock his Bankai. With further training, he can fully and completely master his Bankai, unlocking an entirely new level of power.
Each release is a new stage of power, with a new 'basic ability.' At each stage, a Shinigami gains one free technique slot, and is able to purchase two more with EXP. Shinigami have the smallest number of technique slots, but make up for this with Kido.