Well, I havn't yet been able to play with the PFX-to-polys as yet, so I'm no help there... sry
BUT, when you open your brush attribute editor window, go to its' help menu, and choose "Help on this brush" or "help on PaintFX" to get a listing of attribute descriptions and usage,
and this will tell you what that checkbox means, and how it works
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com