This script generates two difference layers from the two layers on the top, A is the original image layer, B the modified (e.g. sharpened) one on top:
A - B = C
B - A = D
A - C + D = B
It's similar to GIMP's built-in Grain Extract/Merge function, but the tonal range is bigger.
Now you can apply further editing and "switch on/off" the differnce by switching the visibility of the subtractive and the additive layer. Also you can change the modification intensity by changing the opacity of these two layers.