Technoids.com

SHSH Blobs are signature files which are verified against an Apple Server to verify an iPhone, iPod Touch, iPad or AppleTV is running the latest version of iOS (Operating System). If for some reason, when you are trying to restore to a previous version, Apple will not allow you to restore it because, you are trying load a older version of iOS. To make this possible, we need to send a request to a different server (Local or designated) which sends a SHSH blob (which was saved by you) back to iTunes faking that its the current version.

As soon as your iDevice is jailbroken backup your SHSH blob(s) with TinyUmbrella. Burn a copy of the SHSH blobs, the iOS ipsw and any jailbreak software used to DVD. SFTP all the www files and custom plists and backup them up as well. If for any reason down the road your iDevice gets bricked and you didn’t backup your SHSH blobs its a nightmare.

Real life example

The iPod Touch I am using as a backup server had iOS 4.1 and worked perfectly. I got keen and pushed the envelope to get more free memory (> 175 MB  free out of 256 MB)
Big mistake I went too far and bricked it. No problem I’ll just restore it and start over. Guess what!  I forgot to backup the SHSH blobs. The only iOS that would work was iOS 4.3.  Just lucky the jailbreak for 4.3.1 was just released the same day !. Got it back up running but wasted a whole day on it The best free memory I could get was 133 MB and had a number of incompatibility issues (the folder structure changed slightly). Had I backed up the SHSH blobs I could have restored the iPod Touch and all content  from backups within half an hour.

NOTE If you upgrade the iOS to try it out you can always downgrade ONLY if you previously saved your  SHSH blobs

Lessons learned
1) If it isn’t broken don’t fix it
2) Backup Backup Backup