Enable Blanking:

Blanking is the name given to the black bars created WinDVD Tweakers Pro itself. You must enable blanking in order to make the Windows desktop and icons become blanked out. You can define custom sizes for each black bar if you disable "Enable Automatic Blanking" You should enable blanking when you have selected all other options like source aspect.

Enable Automatic Blanking:

When enabled WinDVD Tweaker will work out where the black bars should go (recommended).