User blog comment:Xilinoc/Ch. 543 - Letters/@comment-10371091-20130629150214/@comment-11051098-20130630122125

Or he could have tracked down and found those Quincy, and bestowed them power, or used some technique or something to "unlock" or "refresh" the power they had as living humans, so that they could use that power again as souls.

Seeing as Shaz Domino said something about Yhwach having "bestowed him" power, it might be something along those lines.