Board Thread:General Discussion/@comment-9307023-20190103034125/@comment-5398287-20190104043056

If it were part of his Bankai, he wouldn't have been able to activate it while using his Shikai then activate his Bankai later. It's more or less an evolution of his Hollowfication powers, last seen in the form of his Hollow mask, where he calls on the form he unconsciously took against Ulquiorra and maintains control over its power and his own actions - the Hollow side of his Zanpakuto, as it were.