The mirror tool will mirror all the selected ducks about a horizontal or vertical axis passing through the mouse cursor.
For example, to mirror a bline horizontally:
- select the mirror tool, either by clicking in the toolbox or hitting alt-m
- check 'Horizontal' in the Tool Options dialog
- select the bline layer
- select all ducks in the bline by hitting control-a
- drag one of the selected ducks a little
As soon as you start dragging, the ducks will all flip about a vertical axis passing through the duck you're dragging. You will be able to drag the ducks around, but the motion will be constrained to only move left and right.