Spectec SDV-841/842 - Manual (Page 22)

Saving Power
NVD allows you to turn off the video display thereby saving precious
power. This might be useful when listening to audio files or when using a
remote display application.
The NVD installation program creates several application icons under the Start
> Programs > Virtual Display. The VD Power toggles the display power
between on and off each time it is executed. Assigning Hot Keys explains how
to assign this icon to a hardware button.
NOTE: This feature is not available on some devices.