The RLE Gaussian Blur plug-in displaces pixels in a radius from a central point defined in the dialog. A higher value will produce a higher amount of blur. The blur can be set to act in one direction more than the other by clicking the Chain Button so that it is broken, and altering the radii.
By altering the ratio of horizontal to vertical blur, you can give the effect of a motion blur.
RLE blurring works best on images such as computer generated images or images that contain large areas of constant intensity such as text logos.