Starting with iOS 5, Apple added some new checks to ensure people stick with the latest version of iOS. This made downgrading to iOS 5 impossible using SHSH blobs saved by Cydia or TinyUmbrella. But now, thanks to iH8sn0w’s iFaith, you can downgrade from iOS 5.1 or iOS 5.0.1 to iOS 5 custom firmware as long as you have APTicket + SHSH blobs saved for it.
How To Save SHSH Blobs Of Any Old Firmware Running On Your iPhone, iPad, iPod touch Using iFaith [Tutorial]
iH8Sn0w, a Toronto-based iPhone developer, released iFaith, a new SHSH blob dumper for all iOS devices which can save SHSH files of any firmware running on your device even if Apple is not signing it.