The full release, Bankai, is generally a player-augmentation passive ability. That means once you activate it the powers take effect on their own. No, not all of them have icons.
To obtain Bankai, first train the bond with your sword to be stronger by using it a lot (yay more slashy). Eventually you will learn how to call out the spirit of your sword to fight it. Fight it to train the bond with your sword very well. If your bond is strong enough, defeating it can result in learning Bankai!