Usage:
Turn on the Hold switch to automatically disable the display and underclock the CPU. Turn off the Hold switch to return the CPU to normal speed and enable the display.

If you push the ANALOG UP button when turning on the Hold switch, it will only lock the keypad like normal hold mode. The display and CPU speed will not be changed, so that you can watch videos etc. with the keypad locked.

If you push the ANALOG UP button when turning off the Hold switch, the display will remain switched off and the CPU will remain underclocked, but you can operate the PSP. To return to normal, push the power switch to the suspend position, or just press the Screen button.

While the PSP is in hold mode, if you first hold down the Select button, you can use the following buttons: Left, Right, LTrigger, RTrigger, Vol+, Vol-, Start.

Installation:
You can install it in the seplugins folder, and make an entry in VSH.txt. Thus it will be active in the XMB. Its also possible to use it in games by adding it to GAME.txt, but some games may crash etc., if the CPU is underclocked to such a low value.

Optionally, its also possible to install it in your PSP's flash0 so that it will work without a Memory Stick. For this you can use FreePlay's "NewBTCFNedit" and enable it for VSH mode in the all the PSPBT?NF.bin files. Add it before vshmain.prx.

Advanced:
If you want to change the clock speed to your own values, you can use a hex editor to change them. The offsets in the file for v3.8 are
0x1264 - CPU
0x1268 - Bus (Should be half the CPU speed or less)

The values should be entered in hexadecimal. The default value for CPU is 3C (60 MHz in decimal) and the default value for Bus is 1E (30 MHz in decimal).

Note that simply using any arbitrary value will not work. You will have to test and see which values for CPU and Bus speed work.

Credits:
Thanks to adrahil for helping with preventing the suspend mode.

Thanks to Booster for the SysCon Hook sample, based on which Hold+ v3.0 was updated.