BRepAlgoAPI_Common::Modified()

Hi!

Can anyone explain the function BRepAlgoAPI_Common::Modified()?

I am using 2 TopoDS_Shapes in a boolean operation and I want to find out which faces of tshape2 are in the resultshape.
This is what I tried, but the list is always empty:

BRepAlgoAPI_Common common = BRepAlgoAPI_Common(tshape1, tshape2);
const TopTools_ListOfShape & listOfShape = common.Modified(tshape2);

Best Regards,
Matthias