User blog comment:Xilinoc/Ch. 603 - What The Hell/@comment-10497166-20141030053927/@comment-24913480-20141030135324

You are half right. In chapter 565, Haschwalth explains how Yhwach can slipt his soul into pieces and implant them in other beings, usually quincies. However, he also mentions that anyone that has fought a quincy during the invasion also received a piece of Yhwach's soul (Byakuya, Ichigo, Komamura,...) and, when they die, that piece will return to him to make him stronger. So, while he can't kill any shinigami with the Auswahlen (though I'm not sure), he can still be empowered with their deaths.