Normal curvature to roughness explanation?

Can someone explain the new (in 4.14) “normal curvature to roughness” setting under forward shading in Material details?

Is the idea to convert normal variation into roughness in order to fight specular aliasing when using forward shading? How does this compare with the Valve technique of baking increased roughness into smaller mipmaps (GDC 2015)? Is this a complete replacement for that, and are there any limitations/downsides to this approach?

How is it working under the hood, is it just something like ddx/ddy?

Edit: I just saw some explanation in the changelog:

Cannot find the option in 4.16, is it still available?

I recreated the effect here
I pulled the function from the basePassPixelShader.usf file