Sometimes recovery mode just doesn’t do the trick and you need to go into DFU mode to reload your i OS. i Folks are often confused about the difference between DFU and Recovery Mode.DFU stands for the Device Firmware Update, it’s that state of your i Device when it doesn’t load the Operating system on itself but still interacts with i Tunes.If you accidentally tried to upgrade your iphone 3GS to i OS5 through stock official IPSW through i Tunes on an already jailbroken i Phone with a baseband of things are not going to be smooth in terms of service and reception and may require a ‘re-flash’ aka a re-install 0f the baseband.
Very Important that you have old boot rom – new boot rom may/will likely kill the GPS functionality on the i Phone.
As part of the i OS restore process, i Tunes has to check with Apple's servers to 'sign' each install of i OS just like App Store apps must be signed to run.
Once a new version comes out, Apple stops signing previous releases to keep everyone running the same i OS version and to prevent you from jailbraking your device again after restoring by forcing you to do an update.
Using Grayd00r, older device users will be able to have a feel of the new i OS 9 UI, multitasking with Slide Over, App Time Machine, which recover old application versions, Control Center, Software Updates, Jailbreak and Cydia.
Last but not least, the ability to improve the speed of your old i Pod touch.