Board Thread:Questions and Answers/@comment-25051789-20140627010334/@comment-24479616-20140627021250

well, in a sense you could say that's the same: Zangetsu 'is' Ichigos power (manifested as a Zanpakuto spirit), so whenever Ichigo uses a great deal of his power, he does this by drawing it from Zangetsu/ somehow interacting with him, and that's represented by the change of eye color.