User blog comment:Njalm2/The Beauty of Zanjutsu/@comment-18812574-20150410193452/@comment-3403804-20150410205922

First off, thanks for making such a long and cool comment, keeps me going. As for the whole Shikai and Bankai compatibility thing, my own understanding is that yes, this would be the case, assuming there is such a thing as a compatibility in the first place. My personal understanding of things like Bankai, and how so few people achieve it, is not because it's so extremely difficult to achieve, it's that most individual's spirits never evolve to the point where a Bankai could exist. Just like any other release of a Zanpakuto, a Bankai is formed from your heart, if your experiences, emotions, goals, strength of will, character and enlightenment are too shallow and incomplete as a person, then a Bankai will never surface. There simply wouldn't be a foundation for it to form on. In my opinion, it's not that the Bankai is kept away from you, you can't obtain a power that doesn't yet exist, after all.

I won't go into your disagreement about the nature of releases, and how they're not techniques, because that's a difference of view, more than likely.

As for the separation between Spirit and Shinigami, I don't actually consider them separate, at least not in terms of them being separate entities. A Zanpakuto Spirit, to me, is the direct expression of your heart and your sense of self. To call it a manifestation is probably more along my lines of thought, what lies in your heart is manifested in the Asauchi, and that expression assumes a form of it's own, and becomes an extension of yourself, even as it is technically separate from you. It's complicated, and hard to explain.

As for your follow-up question about how it works in practice, I'd require a more detailed question, provide an example perhaps?