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