What is one? Well, it's like a flash-back, but it's a flash-forward. It's a scene at the beginning of the book, that is actually a scene of a much later part of the book. It uses the tension/action of the later scene to hook the reader right at the beginning. Twilight does this in the "Preface" when Bella is talking about how she never gave much thought to dying and then it goes on to Chapter 1.
The good thing about starting with a flash-forward is that the book is the tension/action to hook the reader. Obvi. The bad thing about them is when you turn to Chapter 1, that tension is gone. If you introduced a fantasy world or interesting tidbit of information, it's gone. Now you have to wait and read through, what feels like preamble but is really the first part of the book, to get back to that interesting tidbit. Plus the reader already knows about that tidbit so when you re-introduce it, there's no twist, no surprise. As the reader you're just like, yeah, I know about that already, now can we get on with the story? If the flash-forward was meant to excite your beginning, instead maybe change the actual beginning to be more exciting. Then the tension can build naturally, instead of having the ups and downs of a flash-forward.
Twilight's flash-forward worked because it was so vague. It didn't give any of the plot away except that she might die at the end. And dying is some pretty big tension. If Stephenie Meyer had mentioned something about a vampire instead of a "hunter" (which is how she writes it in the preface), I don't think it would have worked as well. She would have given away too much of the plot, since Edward being a vampire is supposed to be the big twist.
There are also some flash-forwards where authors use scenes from the very beginning instead of later in the book. I've read two books where the flash-forward was in the first scene. It made no sense to me to have included the flash-forward, because I read it a few pages later. I don't really know why the authors did that, but it kinda turned me off because I had to read the same scene twice in a row. I ended up just skipped the scene the second time around.
I admit I have a flash-forward in my WIP and I've been toying with it for a long time, deciding whether or not to keep it in. It's pretty vague, but I think it may give away too much of one part of the plot, though it sets up other parts of the plot really well. So I have more deliberating to do.
Anywho, I think the point is, flash-forwards can be hard to pull off. Keep them vague, but suspenseful, and then the actual beginning has to be just right, otherwise you can lose a lot of the tension.
How do you feel about prologues/flash-forwards/
prefaces? Useful? Suspenseful? Or a let-down once the story actually starts?