This is one of new KFX manga features. There is option to enable it in book settings directly on Kindle.
http://www.youtube.com/watch?v=MEoNXG80iSY&t=0m25s
That box on bottom on double spread pages.
EDIT:
OK. I got it.. partially. As expected this feature is controlled by
page-spread-* properties in OPF spine and require unlocked
orientation-lock. But that is not enough. It is somewhat connected with size of both images. If they pass some magical limit preview kick in.
I don't know that limit yet.
Additionally books created by Kindle Comic Creator don't trigger it too.
Publishers of this type of content must have access to some additional specifications as
Amazon Kindle Publishing Guidelines just don't describe it at all.