Board Thread:General Discussion/@comment-4957813-20131113141215/@comment-11051098-20140208191610

The medallion stole the Bankai, but the Shikai appeared to be unaffected. The way I interpret it, is that the "top layer" of the Zanpakutô's power, the power that goes beyond Shikai and makes it possible to enter Bankai, was the part that was stolen.

Seeing as Hitsugaya couldn't feel Hyôrinmaru's spirit, the solution could be that the Zanpakutô spirit became impossible for its owner to feel because a part of the spirit and/or its power was stolen, and the medallion could have allowed its owner to use the Bankai, but not being able to sense the Zanpakutô spirit.