Unsharp masking is a technique in that an unsharp low contrast and low density positive is made of the camera film negative.The compression of the original camera negative density scale is directly proportional to the peak density of the mask.
The method that I use is to place a sheet of ortho lith film (Freestyle or Photo Warehouse) emulsion side down on a sheet of black paper. (this is important since this film has no antihalation coating). On top of this sheet of lith film I lay a sheet of fixed and unexposed film or clear acetate. On top of the acetate I lay the camera negative emulsion side up. On top of this is placed a sheet of Duratrans or similar untextured diffusion material. This is then exposed with the enlarger. Typically with my Saunders 4550 VCCE with the enlarger head at 8X10 enlargement the times will be about 16-20 seconds at F16. The lith film is developed in Dektol 1-30 for 2 1/2 to 3 minutes, fixed and washed. The mask peak density should be .15 to .35 This mask is best evaluated by laying it on a sheet of white paper. To evaluate it by light transmission does not work well since the density of the mask is so low. If one does not have a densitometer the density can be evaluated by comparison to a .30 ND filter.
The effect is obtained by printing the mask in registration with the camera negative. The registration is not ultra critical since the mask is unsharp.
Because the overall camera negative density range will be compressed by the amount of the peak density of the mask, the contrast grade of the paper/filtration can be increased without exceeding the papers exposure scale. This has the effect of increasing local contrast and hence a greater "glow" from the image. Another benefit of an unsharp mask is that it increases apparent sharpness due to edge effects.
I don't have anything currently posted depicting the effects of this practice. I have had images posted in the past that have had unsharp and also sharp masking utilized.