The books are sent before the publishing date and all they ask is that you don't do any book reviews prior to the publishing date and that you sync your reading data as you progress the ebook. Plus a small survey at the end giving your opinion of the book.
I've had some really good books over the time I've been subscribed, some by new authours and some by established writers. Normally they send an email and you are given a choice of one or two books you can download.
For anyone who is interested in getting free ebooks, give it a try.
There is a Highway to Hell but only a Stairway to Heaven