In the old days you could downgrade your phone just by putting it in dfu mode and iinstalling an older version of the OS because apple would sign the file and encrypt it. They no longer do that for oder versions so by saving your file to the cydia server you can redirect the signing request to the cydia server and fake itunes into thinking it was done by apple...allowing you to isntall the oder version. That is why anyone that had saved their file could downgrade but anyone that didnt....was out of luck when 4.02 came out.