Jargon Buster – What Is A SHSH File?
What is a SHSH file? In all newer iDevices, when restoring firmware files through iTunes, iTunes requests a certain file, (a kind of authorization). This allows Apple to deny downgrading to firmware files that they no longer support.
This is bad news for jailbreaking users. If you upgrade to a newer firmware and then find you cannot jailbreak it, you will be stuck, unless you have your SHSH files for the firmware you need to downgrade to.
So basically, in summary, saving SHSH files is an amazing idea! It allows you to downgrade to all previous firmwares provided by Apple.
For a full tutorial on how to save your SHSH files using TinyUmbrella, click here. Also, for a full tutorial on how to downgrade using saved SHSH files, click here if you are using Mac OS X, and click here if you are using a PC