if you search for a downgrading guide, you'll see that after the garbled text is displayed, you have to wait some < 30 seconds and then do a hard reboot.
after that the psp is tricked to 1.0, so you can use a 1.5 upgrade file to effectively downgrade
edit-didnt see that last part, you have to hold down the power switch for some 10-15 secs
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