A Magic Memory Stick is the counterpart to the Pandora's Battery. You make a MMS and a PB on a PSP that already has a custom firmware, then you put them both into a PSP that does net yet have a CFW in order to downgrade it to a firmware (usually 1.50) you can easily upgrade to a CFW. But, depending on your current firmware, all that might not be necessary. There are much less complicated methods of downgrading for older firmwares. I'm not entirely sure if this is true for the PSP-1000... The Half Byte loader was recently released, too, but with minimal homebrew support and don't even think about ISOs yet. Still, there's hope it'll lead to a downgrader or some such.