The terms and conditions are here:
https://kdp.amazon.com/self-publishi...=APILE934L348N
I see that an author's share of the pie will be based on their share of total lending in the prime program. If there are a total of 100 checkouts of prime books in a month and a particular book is checked out one time, then the author of that book will get one percent of the total. In reality, I expect that most of the money will go to a handful of books.