User blog comment:Xilinoc/Ch. 681 - THE END TWO WORLD/@comment-11293956-20160707191829

So for those that are wondering how exactly Book of the End allowed Orihime to fix Tensa Zangetsu, I might have an explanation. When Tsukishima cuts people with Book of the End, he has the option of replacing people in the victim's memories (he did this when he cut Orihime; in her memories, he replaced Ichigo during the events of the Soul Society Arc).

"But...if there was a past where that didn't happen...then you should be able to reject it there."

This quote from the chapter leads me to beleive that when he cut Ichigo, he replaced Yhwach in Ichigo's memories, meaning that The Almighty wasn't used to break Tensa Zangetsu.

With this explanation though, there are a few things that bother me. -Why didn't Yhwach see this coming? -If he did see this coming, why doesn't he care? -Apart from having it reforged by Oetsu, are Orihime's powers the only thing that can fix a Bankai?