A real question that needs to be asked is, why are the stolen Banka's still working with their users dead. Does the medallion somehow trap the Zanpaktou spirit inside and subdue it? I remember Hitsugaya trying but failing at his attempt to talk to Hyorinmaru.....What is the implication of this given that Zanpaktous are part of their owner's spirits?
Well, how I understand it-
Zan Shikai, and Bankai spirits are different (so they can steal bankai spirits, but shikai spirits stay with their original 'lords'), and well, they're a part of the shinigami's soul, but they have their own mind and everything, so the medallions normally capture and subdue them as you said.
LOL Well, I can't think of a reason. This just confuses me. I never understood it from Tousen's days...a Shinigami is dead and their weapon (born from their soul) can be used by someone else.
When i think of Zangetsu's Bankai form, it was different from Shikai, but it was still Zangetsu (memories, experiences, etc). So how you can have bankai stolen and retain shikai baffles me. It would be nice if they only stole the power of the bankai and cut off communication with the spirit. To actually steal the spirit would not be a good explanation IMO. I had hoped stealing Bankai's depended on how in tune you were with your zankpakutou so naturally Ichigo's couldn't be stolen because him and Zan are buddies. But after seeing Yamaji's get taken i guess not...after thousands of years if you and you're Zanpakutou aren't friends I don't know what to tell you.
Anyway i went a bit off topic, sorry. RIP Yama-ji and hopefully Kubo will explain the bankai stealing soon.
I think the medallion works in principle similarly to mutilation. The shinigami is by all intents and purposes a soul made of spiritrons however we also know that shinigami have souls (basically a soul's soul). As far as we have seen the zampakuto spirit lives in an inner world which is by no intents or purposes real (otherwise there would be a tiny room-organ thing inside shinigami with an even smaller person inside). So the spirit exists within a shinigami as a part of his soul which would have the implication that the spirit itself is not something quite physical as far as the shinigami is concerned even if the inner world appears to mimic the real world. Anyways, the badge is clearly having an effect over the zampakuto spirit itself from what we have seen. So my idea is that the badge simply rips the power out of a zampakuto spirit in a way in which it would be in principle similar to mutilating someone since the power taken is basically an inherent part of the zampakuto spirit who is in itself a representation of the shinigami's power which leaves behind a maimed power without bankai. As far as the science behind all of this, I don't think the manga will elaborate on this.
I think the reason they are able to steal Bankai is because in Shikai,the Zanpakutou doesn't materialize;it is still "fused" with it's owner.We do know,however,that for Bankai one needs to materialize the Zanpakutou,and if you think of Bankai simply as the Zanpakutou materialized(which is materialized,basically not part of the Shinigami anymore),it would be just like stealing someone and using them as pawns.(I remember how Renji's Zabimaru appeared when he went to the underground training area where Ichigo was training for Bankai;the medallion just steals and contains inside itself the Zanpakutou,kinda like kidnapping someone).
So "if you release only your Shikai you'll be safe. Release the Bankai and all you will have is a Asauchitsu".
From Quincies 101 or Fighting Quincies for dumbles, title yet to be decided.
I am thinking something of a soul split control sort of. I saw parts of the rogue zanpaktou bleach bit and it seem that the zanpaktou is sort of a piece of the soul, not the entire part. I believe the medallion is sort of a prison for the zanpaktous full release form.
I also believe the breaking of the medallion releasing the Bankai is part of a fan service in the future when Byakuya will be let back as his Senbonzakura was let free by the Captain Commander.
This is ofc all just taken out of my rare end but some substance is stuffed up there too