Also what do you mean for difussion??
Most people are familiar with using a difusing device such as a soft focus filter or a hairnet in front of the camera lens. Even if one hasn't done it, we've all seen the effects - it scatters bright light into darker areas, so there is a sort of halo around bright subjects.
If you use a diffuser in printing from a negative, between the enlarger lens and paper, you're doing the same thing, but since the light source is a negative, you're scattering light from bright parts (shadows on the print), into dark parts (highlights on the print). This creates a sort of dark halo around dark parts of the subject. It's a much less familiar look than the bright halos.