I believe you are talking about the button that releases the battery cover, so you can get to the PSP's battery.
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Just push the button which you close/open your PSP Down.
what button on PSP skips the initial setup
It is the button that changes the brightness of the screen. If you don't know where it is, then check your PSP manual.
The AOSS button is something on a wireless router, it's not on the PSP itself.
The hold button neutralizes all other actions on the psp. If the hold button is turned on nothing you press will do anything. Its useful for transporting your psp and having a game on hold for a bit.