GeekGrade uses many exploits together to create a tethered downgrade without SHSH blobs. They happen in this order:
1.limera1n - the bootrom exploit that allows pwned DFU Mode. Pwned DFU mode puts the device in a state where custom firmware files can be falsely checked as legitimate when iTunes restores to a custom firmware
2.Custom firmware - This only works because iTunes originally only asked the device if the firmware was legitimately from Apple. Apple firmware has specific img3 hashes, and iTunes asks the device if it will accept them. Pwned DFU mode allows the device to 'say yes'. The custom firmware uses SHSH blobs "donated for the project" and although they don't match the device, limera1n bypasses iOS' verification for correct blobs.
3.iTunes 11.0 - All iTunes versions below 11.1 can be exploited with pwned DFU mode to restore to a custom firmware. This is because iTunes previously only asked the device if the hashes were correct. All versions above 11.0.5 will recalculate the hash and check it again before restoring.
Exploitable Devices and Compatible Firmware
iPhone 4 (iPhone3,1) - iOS 6.1.2 - iOS 6.1 - iOS 5.1.1 - iOS 5.1 - iOS 5.0.1 - iOS 4.3.5 - iOS 4.3.4 - iOS 4.3.3 - iOS 4.3.1 - iOS 4.1 - iOS 4.0.2
iPhone 4 (iPhone3,2) - iOS 6.1.3
iPhone 4 (iPhone3,3) - iOS 6.1.3 - iOS 5.1.1
iPod touch (4th generation) (iPod4,1) - iOS 6.1.3 - iOS 6.1 1 - iOS 6.0 - iOS 5.1.1 - iOS 5.1 - iOS 5.0.1 - iOS 4.3.5 - iOS 4.3.3 - iOS 4.1
iPad (iPad1,1) - iOS 3.2