The Mirror Tool will mirror all the selected Handle about a horizontal or vertical axis passing through the mouse cursor.
Horizontal : Horizontal axis mirror.
Vertical : Vertical axis mirror.
For example, to mirror a spline horizontally:
select the Mirror Tool, either by clicking in the Toolbox or hitting
Horizontalin 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.