according to the docs, Brep.Trim(Brep, Double) should remove parts that are in normal direction of the cutter. However, if used with a flipped Brep as cutter, the opposite part is removed.
Example: TrimBrep.py (1008 Bytes)
IMHO this is either a bug or should be clearly stated in the docs.
Workaround: An additional BrepFaceList.Flip(True) gets rid of the flipped state and leads to the intended results.