When you update your iDevice, iTunes downloads a firmware file. That firmware file is a generic ZIP file. If you extract it, you will find many files. The biggest one is the filesystem (A.K.A. the one that is installed on the device). It is encrypted. If you run it through iDecryptIt with the correct key, iDecryptIt will decrypt it. The decrypted file can be extracted with 7-zip or mounted as a virtual hard drive with certain tools.