Board Thread:General Discussion/@comment-10587310-20130620070434/@comment-6007020-20140118054328

Salindy13 wrote:

Oh, I remember now. I was just thinking that because I'd never seen Renji's Bankai get stolen yet. This makes me wonder how those pills work. I thought the original owner had to touch the pill in order for the Bankai to return to them. Unless Urahara snuck himself into 0 division to hand Byakuya a pill which is unlikely.

The pills work by inserting Hollow energy into the user's soul, thus triggering a mini-Hollowification. Since one's Zanpakuto is connected to the soul, the stolen Bankai thus becomes tainted with Hollow energies. Said energies are deadly to the Quincy who steal them, so they are forced to return them or be destroyed. Side effects include (but are not limited to) personality changes in the Zanpakuto Spirit you know and love, and creepy masks growing over your face.