Hi!

http://www.oopo.net/consoledev/files/psp-updating.txt

See text:
"This PBP format looks like an ordinary archive, much like a tar file, used to store a collection of files. Basically a simple header".

See also text:
"I flashed my PSP with the possibly-fake update file. It was accepted as valid and installed fine, but when it was finished my PSP decided it doesn't want to do anything but sit there and look for something on the memorystick. So, do any flashing at your own risk".

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My advice:

Unless you need a new paperweight don't play with this feature.
(You will BRICK the Sony Playstation Portable).

Luciano