Board Thread:General Discussion/@comment-26475399-20151228215405/@comment-3423279-20151230030523

well, my theory on Final Getsuga is that it doesn't just take away their powers if used, but it also removes their Zanpakutou spirit as well, something that has always been with them even without any Shinigami training.

Essentially what the Final Getsuga does is grant the user immense power but at the cost of quite literally carving their soul apart.