Ddy gives the partial derivative of any input relative to screen-space y-coordinate. https://docs.microsoft.com/en-us/windows/desktop/direct3dhlsl/dx-graphics-hlsl-ddy
In the pixel shader you can use it to obtain the surface normal by taking the cross product of ddx and ddy of the world position. An example can be seen here Flat shading/low poly .
The culling mode is not changed. Just making an effect that is rendering the ddy of the worldposition of a flat surface will show that the value is flipped if it is rendered to a render target compared to just rendered to the backbuffer. Perhaps the coordinate system is flipped internally when using render targets?