StarTech.com USB2DVI - Manual (Page 8)

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Configure the secondary display by selecting it from the dropdown
menu, then adjusting the desired settings:
Using the Second Display in Extended Mode
In Extended Mode, the monitor that is connected to USB2DVI creates
an extension of the Windows Desktop displayed on the primary
monitor. To set the display to function in this mode (once selected
from the dropdown menu), check the box marked Extend my windows
desktop onto this monitor, then arrange its position (virtually) with
respect to the primary monitor by dragging the monitor icons (shown
above as Primary/Secondary orientation) to reflect the physical
orientation of the displays.
Using the Second Display in Clone Mode (default)
In Clone Mode, the monitor that is connected to USB2DVI simply
mirrors the image displayed on the primary monitor. To set the display
to function in this mode (once selected from the dropdown menu),
uncheck the box marked Extend my windows desktop onto this monitor.
Designating the Primary Display
To select which of the connected monitors will act as the primary
display, select the monitor that will be used as the Primary Display from
the Dropdown menu (referenced on page 4), and place a checkmark
next to Use this device as the primary monitor. On some displays it is
necessary to disable the main display (i.e. uncheck the box marked
Extend my Windows display onto this monitor for the main display).
Following this change, the monitor connected to USB2DVI will remain
set as the primary display even if the computer enters hibernate
mode or operation is suspended or powered down. If the monitor is
disconnected from USB2DVI, or USB2DVI is removed from the host
computer, the original monitor will revert to being the primary display.
Alternate Configuration
Following the software installation for USB2DVI, you may have noticed
an icon in the taskbar as illustrated below. Clicking on this icon will
open a menu that offers configuration of the display (connected to
USB2DVI):