This is an analogical procedure called Sabbatier Effect, it can also (and more easily) done in PS, but, the results are not this good. It is said that you shouldn't perform it on portraits, due to the change in eyes and skin color, but I think it looks pretty well, not in all cases though.
How is it made? Check your time for the print photograph, put it in the developer until you begin to see the forming image, then take it out quickly and expose it to the enlarger light untl the white parts look grey, then you can go back to the developer and end the procedure normally.