Another neat feature with FMGS 47.0 is the inclusion of Terrain and Weather dimming functionality. The A320 Display Unit controllers has a ND dual pot — the outer (larger) switch controls the ND screen brightness. The inner (smaller) switch controls the weather or terrain brightness. Corrected – Outer knob controls the weather or terrain overlay, inner knob controls screen brightness (thanks Chris). 

Potentiometers can be setup with the FMGS Configure program and calibrated under ‘DUs Dimmer Potentiometers’ dialogue box.

Vu has a short video showing us weather dimming functionality:

And a video of terrain dimming functionality: