One wag wrote: "Can't explain it strictly in terms of code. Roughly, Photoshop calculates a 'low pass' of the image, finding the regions of 'flat' color (anything that wouldn't be considered an edge). It multiplies the result of the low pass with a blur (likely Gaussian) and the active selection. It is probably safe to assume that the radius parameter applies to both the low pass and the blur, while the threshold parameter only affects the low pass."[
^]
For the technical details (which are far "beyond me"): see:[
^].