Asus P9X79-E WS - Manual (Page 176)

5-14 Chapter 5: Multiple GPU technology support
B3. The NVIDIA Control Panel window
appears.
Enabling SLI settings
From the NVIDIA Control Panel window,
select Configure SLI, Surround, PhysX.
In the Quad-SLI enabled, click Maximize
3D Performance SLI to set the display for
viewing SLI rendered content. When done,
click Apply.