As far as I can tell, it’s still a private programme where Amazon manages everything in-house. If I’m not mistaken, they pick selected partners (publishers, authors) and designers/illustrators.
There’s no further info yet. Educated guess: they’re gauging readers’ interest and collecting feedback at the moment.
There’s also a possibility they want to keep it private, à la Kindle Singles, because they don’t want quality to suffer. This is one of the reason I decided to focus on the boring stuff, which could expand to KFX in some way.