Smart Contrast is a form of
Dynamic Contrast.
The idea is to turn down the backlight on a TV/Monitor when the image is dark and turn it up for brighter images. This can help for some users to get darker dark scenes, but it kind of gets lost when you have a scene with both. It may also make game interfaces look dark in darker scenes since it dims the full backlight on most screens.
Many feel this dynamic contrast was just make to inflate numbers since you can take a screen with an 800:1 contrast ratio and claim 20 million : 1 with dynamic contrast.
I personally do NOT use it and mine is only to go from 1000:1 to 1500:1.
This image gives you an idea of what I am talking about with Dynamic contrast and the fact that it can alter...