When you FD, there are three conditions that'll cause a memwipe:
1. small random chance of instant memwipe on mob (using sneak before standing seems to help)
2. the mob paths back and resets at their spawn point (this is what makes roamers difficult... it can take a long time for some of them to reset)
3. stay FD for a full 2 minutes, you'll get the "enemies have forgotten you"
When you are trying to split mobs, you can leverage #2 to split mobs (e.g., wait for one to path back and reset)... as an SK, snare will be useful for splitting.
If you want to learn more about FD splitting, I'd recommend reading some of the posts/guides in the forums at
monkly-business.net (under Guides and Tactics)