FWCF is a separate flash partition for all changes made to the /etc/ directory. There is a small tool named fwcf, which is used to setup the system or to commit changes to the fwcf partition.
On bootup a script is executed, which calls fwcf setup to setup /etc/ as memory filesystem and overlay the changes committed to the fwcf partition.
If you change anything in /etc/ and like to keep the change, it is required to execute fwcf commit. This will compress all changed or new files in /etc/ and write the result into the fwcf partition. The fwcf partition is 256 Kb in size. This size is not changeable at the moment.
If you need more detailed information, please read the specification of FWCF.
If you want to remove all your changes and start your configuration from scratch, use fwcf erase. This is also required if you switch between compression plugins. Right now LZO plugin is default.