User blog comment:Xilinoc/Ch. 540 - The Sword Five/@comment-7488559-20130605204755/@comment-3177613-20130606093830

I believe that Ichigo's Quincy powers (Yhwach), have taken over his Shinigami powers, hence why Yhwach was able to teach him Getsuga Tenshou and Bankai, and tell him his Zanpakuto's name. When Ichigo activates his Bankai, the power balance shifts, making Zangetsu, now stronger than Yhwach, wear his true form, the young man we know as Tensa Zangetsu.