Apply symmetry on all curves or the selected curves.
The curves are mirrored across the symmetry plane (a semi-transparent gray plane).
You can switch the normal of the symmetry plane to
z axes by tapping the corresponding buttons on the left toolbar.
You can move the symmetry plane using the transform widget.
You can also reverse the normal of the symmetry plane by tapping the reverse button on the left toolbar.
When applying symmetry, every curve on the negative side of the symmetry plane is removed.