code: public void onTentativeMove(GeometryEditTentativeMoveEvent event) {
  		try {
  			Coordinate[] vertices = editService.getIndexService().getSiblingVertices(editService.getGeometry(),
  					editService.getInsertIndex());
  			String geometryType = editService.getIndexService().getGeometryType(editService.getGeometry(),
  					editService.getInsertIndex());
  
  			if (vertices != null
  					&& (Geometry.LINE_STRING.equals(geometryType) || Geometry.LINEAR_RING.equals(geometryType))) {
  				Coordinate temp1 = event.getOrigin();
  				Coordinate temp2 = event.getCurrentPosition();
  				Coordinate c1 = mapPresenter.getViewPort().getTransformationService()
  						.transform(temp1, RenderSpace.WORLD, RenderSpace.SCREEN);
  				Coordinate c2 = mapPresenter.getViewPort().getTransformationService()
  						.transform(temp2, RenderSpace.WORLD, RenderSpace.SCREEN);
  
  				tentativeMoveLine.setStep(0, new MoveTo(false, c1.getX(), c1.getY()));
  				tentativeMoveLine.setStep(1, new LineTo(false, c2.getX(), c2.getY()));
  			} else if (vertices != null && Geometry.LINEAR_RING.equals(geometryType)) {
  				// Draw the second line (as an option...)
  			}
  		} catch (GeometryIndexNotFoundException e) {
  			throw new IllegalStateException(e);
  		}
  	}
docs: Renders a line from the last inserted point to the current mouse position, indicating what the new situation
  would look like if a vertex where to be inserted at the mouse location.