User blog comment:StonerCatJay/Rereading the series- theories and speculation/@comment-3230060-20140522160420/@comment-24963493-20140523000259

I checked out his fight with White and it doesn't mention it. He doesn't call out any attacks, he just releases his zanpakuto and uses flames as he attacks. However, when teaching Ichigo about the Final Getsuga Tensho, he references Engetsu when stating how Zangetsu will react. He could just be talking about the final version of whatever special attack he has, but since the only one we see is Getsuga and given the context, it's most likely a shinigami move.

It could be a Vollstandig disguised as a bankai, that'd be really interesting. Though I don't believe anyone with a piece of Yhwach's soul can use it, since it's stated that there are people on both sides of the war who have pieces of Yhwach's soul within them, and we don't see any shinigami exhibiting Quincy powers (other than Ichigo). I think that Yhwach can give out pieces of his soul with or without turning people into Quincy, all Quincy automatically have a piece of his soul inside him, and When he engraves a letter into the piece of soul he gives you, you become more powerful. I'm pretty sure that any Quincy can use it, though we've only seen Stern Ritter using it. Since Ichigo's a Quincy, he should be able to use it.