It's the HWSync this syncs the redraws to the refresh rate of the display. This isn't on by default because it is unstable under Maemo, it can cause your GUI to hard-lock after a while (you'll still be able to SSH in if SSH is running and you're connected) forcing you to reboot. This affects my N900, but doesn't seem to affect everybody, so feel free to try it out, but it was disabled by default for good reason.