When does Oracle use 'Index Fast Scan'

[Ryan]

I have found that the vast majority of time that Oracle chooses this method, my statistics are stale and the query is sub-optimal. One time, Oracle changed from a 'range scan' to this type of scan with a FIRST_ROWS hint and this reduced performance.   This is just a full scan of the index, one block at a time right? When would this ever be superior to a Fast Full Scan or a Range Scan?