FBF stands for Frame By Frame, and is the process of advancing N by a single frame at a time by clicking when caps lock is deactivated in Ned.
This technique is used by many members of the community. In the highscoring community its used to test the possibilities of the levels, see how much is left until they're maxed, try new routes, etc. On NUMA, its also used by a lot of players, because they just aren't in the mood of playing realtime at that moment, or some of them have become good with fbf and they prefer to post those demos. However, you must remember that, if you're posting fbf'ed demos, you must always indicate so. If not, it usually leads to confusion. For example, all runs by miststalker are always using fbf unless he indicates its realtime (just the opposite you should do).