Wren,
Sales rank is determined AFTER you go free. I was #12 in Free fiction overall, and my happy butt was dumped right back at #249,000 overall in paid kindle when the ride was over.

As other books came off being free, my sales skyrocketed to #1200 in paid Kindle, #74 on Women's Fiction. But I know that doesn't happen to every book that goes free.
Returns. When free, anyone who bought your book recently can return it to get it free. Amazon will give you the lower price of a book in the last 7 days. Also, people who meant to borrow it, but bought it, will return it and fix it.

I have 14 returns making my sales 165 for the month so far, and 21 borrows.
HTH.