Mirrors the path along the line between its first and last vertex. This allows you to create symmetrical shapes in Ae easily.
If you apply the expression to a path, it mirrors the shape along the line between the first and last vertex.
As soon as an expression is applied to a path, Ae does not allow you to modify it manually anymore unless you deactivate the expression again. In order to still be able to get a life preview of the mirrored shape while you are changing it, you can apply the expression to a duplicate of the shape that is linked to the original. Then you can edit the original and the duplicate with the mirrored shape updates instantly when you change something. See the tutorial below for details.