The idea is that you could select a range of edits in the undo history, then either through a context menu or a dedicated button, you could keep the edits before the selection on the original layer, then the selected edits onto a new layer, and the ones after the selected onto a third layer.
This would be a pixel diff, so if a brush used a multiply blend mode, it would just replace it with whatever the pixel value ended up as.
I know it would be very useful for those times when you draw the lineart on the wrong layer or something like that.
I personally want to work on implementing this.