User blog comment:Xilinoc/Ch. 686 - Death & Strawberry (End of Bleach reMIX)/@comment-28131457-20160818150734

Just wanna share with you guys, a commenter(Exe-Saviour) at Reddit theorized what actually happened during the Yhwach reiatsu thing disappears once Ichigo's kid touched it. He theorized that it was NOT Kazui that destroyed Yhwach's remnant, it was instead Ichigo, back in the past when he was delivering the final blow to Yhwach.

Pretty much, Yhwach's last moment of the Almighty, he uses it to find the happiest moment in Soul Society and Ichigo's life, and as he can alter the future, he sends his presence to the future in an attempt to kill Ichigo, his kid, the rest and destroy Soul Society. But Yhwach dies by Ichigo in the past before that moment can come to realization, so his Almighty ceases to continue, and his remnants disperse harmlessly in Soul Society and near Kazui's hand.

I actually prefers this guy's theory, over the speculating of how Kazui destroyed it.