so can i upgrade to iOS4 then down back to 3.1.3 ??
I wasn't able to, nor could I erase 3.1.3 tethered with a normal 3.1.3 then use Spirit for untethered. Just doesn't seem to work...used many dif computers, versions of itunes, followed many steps and directions on how to do it. Nada.
Basically they're little security keys that Apple double-checks when you want to install a new firmware. Since Apple will only let you update to the current firmware, you want to save your blobs (aka SHSH, or SHSH blobs) for each firmware while Apple is still signing them. This is what Make My Life Easier does--or you can use the TinyUmbrella program to save them to your computer.
An example: I was on 4.0.1 and wanted to update to 4.1. At that point Apple was only allowing updates to install 4.2.1. BUT since I had my 4.1 SHSH saved, I was able to load 4.1 instead. Without them, my only option would have been to update to 4.2.1, which at that point was only available for a tethered jailbreak--going to 4.1 let me stay untethered.