This effect permit to flip horizontally and/or vertically images in the current layer (see the examples below). This effect does not use or require FX Stack keyframes.
Source ImageVertical FlipHorizontal FlipHorizontal and vertical flip
The Distortion > Mirror effect
This effect allows you to do a symmetry in relation to an axis in the current layer.
* The Pivot X, Pivot Y and Angle numeric fields allows you to define the axis the symmetry will be applied to.
* When using FX Stack keyframes, the Rotation parameter reflects the number of complete rotations to be carried out before arriving at the chosen angle.
* The Opacity parameter allows you to adjust the transparency of the reflected image.
Source ImageMirror effect, opacity 50%
If needed, you can move the axis through the HUD interactively in the current project window.