Contrary to the other blur plug-ins, the Selective Gaussian Blur plug-in doesn't act on all pixels: blur is applied only if the difference between its value and the value of the surrounding pixels is less than a defined Delta value. So, contrasts are preserved because difference is high on contrast limits. It is used to blur a background so that the foreground subject will stand out better. This add a sense of depth to the image with only a single operation.
Here you can set the blur intensity, in pixels.
Here you can set the maximum difference (0-255) between the pixel value and the surrounding pixel values. Above this Delta, blur will not be applied to that pixel.