I would like to obtain an effect where pixels get spreaded locally. I tried to reproduce how such a filter would look, I hope you get the idea:
Note: here I ran a blur, aded noise and then added a lot of contrast to give a "spreading effect" and remove the noise in plain areas. But it wouldn't work with not-extreme colours (in the sense of contrast).
Edit: now that I've found the answer, here the official definition for the filter which describes best the effect:
The Spread filter swaps each pixel in the active layer or selection with another randomly chosen pixel by a user specified amount. It works on color transitions, not on plain color areas. No new color is introduced.