The force IP feature allowes you to force Xolox to use a specific IP and port. I found that it's primary use is to resolve conflicts when port 6346 is already being used by another program.

You also will not get many uploads if you are not on the network alot. If you want uploads, you need to stay online for extended periods. Preferably 24/7. That way even if you do have stuff poeple aren't demanding, you will still get your IP out there.