News from Torch:

What does Hold+ do when the switch is enabled?

* Switches off the LCD backlight.
* Switches off the actual LCD screen.
(This is important because the backlight and actual screen are two different things. You can have an image being displayed on the screen while the backlight is switched off, such that you can see the image if you use a flashlight. You can also have the screen switched off while the backlight still illuminates the powered off screen.)
* Underclocks the CPU to 61MHz
(Sony has changed the clock speed functions such that only certain combinations of speeds work correctly. Simply trying to underclock to arbitrary values will result in the CPU simply running at the stock speed. I have verified that Hold+ successfully underclocks to 61MHz.)
* The original screen brightness and clock speed are restored when the Hold switch is released.
* Prevents the PSP from going into suspend mode if you accidently push the power switch too far when turning off Hold mode.
* Allows operation of the PSP with the display turned off, to change volume, skip songs etc.

Release notes:

Added feature to allow operation of the PSP with display turned off.

http://forums.qj.net/showthread.php?t=141671