Bankai Mastery

Bankai Mastery (卍解習得, Bankai Shūtoku) refers to the "completion" of the Bankai form. It is a state of mental and physical perfection of the core Bankai state itself, generally taking a normal Shinigami several decades before they are capable of accessing the form. However, these restraints can be surpassed depending on the skill of the Bankai user.

Overview
When accessed by Ichigo Kurosaki, who had attained the form unintentially, the transformation was due to him having remained in the Bankai state for a time length of roughly three months. In normal cases, Bankai emits an immense spiritual pressure which if not handled with delicacy, can cause the body of the Bankai user to eventually downgrade and weaken, such was the case with Ichigo during his first use of Tensa Zangetsu. However, by remaining transformed for extended periods of time, the Bankai user can desensitize themselves to the negative effects of the transformation, coming to a "peaceful" state of the Bankai form where they can effortlessly regulate their release of energy. Reiatsu output is reduced to an almost negligible point, so much so that it can no longer be sensed by an opponent, having instead been added to the overall reiryoku of the user, allowing that as well to be utilized by the user. At that stage, the Bankai user is said to have "transcended", so that only those who have reached a similar level of power would be capable of sensing their reiryoku. By mastering Bankai, the Shinigami is able to retain the Bankai transformation almost subconsciously. In requiring less spiritual energy to contribute to maintaining and stabilizing the Bankai form itself, the maximum amount of spiritual energy can thus be focused into techniques and stamina use.

Seireitou described it as saying normal Bankai users are unable to fight at maximum power because a large chunk of their energy is used simply to maintain the Bankai release without presenting harm to the Shinigami user. However, by conquering those problems, the full extent of their energy is released and the Bankai will sometimes take on a new form to reflect the maximum reiryoku being utilized. The key to this state is reiryoku conservation rather than boosted reiatsu output. This results in a Shinigami who is able to fight longer and more efficiently without the need to release a larger amount of reiatsu. Ideally, they can even remain transformed for days without returning to their sealed form as long as they do not expend too much spiritual energy, essentially making the Bankai become the Shinigami's base form.

It is said that those who have reached this level of Bankai are immune to the power of the Bankai Ensnaring Device.

Possible Ascension
Seireitou noted that those who could reach the peak of Bankai Mastery could possibly transcend beyond the normal Bankai release to higher levels of power, due to being able to access every ounce of the Shinigami's internal reiryoku without it being expended to maintaining the Bankai.

Notable Users

 * Ichigo Kurosaki

Behind the Scenes
This was inspired by the Full-Power Super Saiyan form featured in Akira Toriyama's Dragon Ball Z series.