This is useful if you have kids and don't want them to mess with anything like the HBC, System Menu, etc. This is mostly used to boot into loaders.
Get the XXXX app forwarder .dol and put it in the root of your SD card.
Here's the link to ForwarderV4.dol used for USB Loader GX:
(You don't need to rename these to boot.dol)
Power up the Wii while holding the reset button to boot into priiloader. Use the D-pad on the wiimote and choose "Load/Install File"
Select the ForwarderV4.dol file to install
Now, go back to the priiloader menu, go into settings, and set autoboot: to installed file.
OPTIONAL: You can set the "Retun To" option to Autoboot, the wii will now autoboot to "installed file".
You can later reverse this by going again to priiloader and set the autoboot option to- System Menu.