This page is dedicated to painting with Symmetrical mode enabled. Not mirror mode, a different page will be focused on pure Mirror mode. And there's also a mode focused on Kaleidoscope mode.
For a first impression, watch this video:
To do this yourself, here's where you enable Symmetrical mode for painting in PD Howler 1:
Painting Faces and Portraits
Here are a few examples created with horizontal symmetry enabled, at one point or another.