Let me explain, without going into too much detail, since I've been warned not to talk out your book in other venues too much, or else you'll never finish it. Right now, I'm in the middle of the second part of the book. My protagonist is going through a major learning experience, and I've already written the final chapter of the section, plus almost all the chapters leading up to it.
Specifically, I've already written what I'm currently calling chapters 7, 8, 9, and 11. The next step was to write chapter 10, which would then finish Part Two of the novel.
The problem was that it turned out that the plot didn't really work the way I had it. One of my first readers (or rather, pre-first readers, since she was wiling to look at some of the work in progress) pointed out that the plot development I had happening in chapter 10 really belonged between chapters 8 and 9.
What to do? On the one hand, I've heard all the warnings about not going back to rewrite until the first draft is complete. But on the other hand, I could already see that chapter 10 was going to feel awkward and out of place. I also knew that certain parts of chapter 8 and 9 were going to need to be changed if I later moved the chapter 10 plot development in between them.
In short, I didn't want to leave chapters 8 and 9 in "first draft" while writing a "second draft" version of chapter 10.
So here's what I did. My first step was to take chapters 8 and 9 and create one Word document that combined both of them. (Aside: I don't know how other writers do it, but when I write a novel I don't keep working in the same document every day, just letting it get longer and longer. Instead, I create separate document files for each chapter, and work only in that file. When it's done, I copy and paste it into a master document, which holds the whole novel.)
I took the new document, called it "Chapter_08-10," and began working with it. Right now, that's where I'm playing in my world. Today, I cut and paste some scenes within the document, wrote a bunch of new stuff, and figured out a better way for the plot to unfold. The writing blahs that hit me Friday have disappeared, replaced by a renewed enthusiasm for the book which led me to write 2000 words today. Once that combined document is done, I'll reassign the chaptering, which might lead to the chapter 11 I've already written being renumbered as chapter 12 or even chapter 13.
An anonymous thank-you shout-out to that first reader whose advice was so valuable, and who brought the first light of dawn to illuminate the plot. Rest assured that your name is already in my Acknowledgments page.