If the truck cannot keep to the right edge of the carriageway, how will its driver indicate that it can be safely overtaken?

In the case of heavy vehicles, oversized vehicles, or those required to observe a specific speed limit, which cannot keep fully to the right edge of the carriageway, they shall indicate that they can be safely overtaken by extending the arm horizontally and moving it repeatedly from back to front with the back of the hand facing rearward, or by activating the right indicator when they do not consider it appropriate to give the arm signal.