Well, it's typically read-only (not enough room), so to actually change anything, you need Fanoush's initfs_flasher.

But it's mounted on /mnt/initfs; if you want to try running things on it, be root, and chroot /mnt/initfs/. There's actually some interesting utilities on there; you can use dsmetest to set the backlight brightness (and much else), and you can use text2screen to dump text.