The options here let you launch Opus or run another command by double-clicking on the Windows desktop. This can provide an extremely quick way to access a Lister when you have no windows currently open - simply find an empty area of the desktop (one with no icons on it) and double-click.
When any of the double-click options are enabled, Opus launches a small helper application called DOpusRT.exe that runs continuously in the background. This is added to the registry to run automatically on startup, and means that double-click on the desktop works even when Opus itself isn't running (DOpusRT.exe will launch Opus automatically if it's not already running when you double-click).
In order to trap double-clicks on the desktop (which runs in another process to Opus) it's necessary to "inject code" into Explorer - if you have an anti-virus or anti-spyware tool installed you may find it detects this and complains. If you find that double-click on the desktop isn't working check if something is blocking DOpusRT.exe from hooking into Explorer or from running on startup.
The Restart DOpusRT.exe button can be used when troubleshooting problems. It will attempt to stop any current instance of DOpusRT.exe and start a new one. It will also tell you if the whole Opus process is running elevated, which can cause problems by preventing messages from Explorer reaching Opus.
If you click the Restart DOpusRT.exe button and find that desktop double-click starts working, but then fails again after the next reboot, one possibility is a conflict with another program which is started after DOpusRT.exe when you reboot. You can use the Delay DOpusRT.exe option to make it wait a specified number of seconds before starting to work around that kind of problem. The exact delay needed will depend on the software causing the conflict.