Shooting with mirrors goes back to at least the Late Great Unpleasantry between Kinfolk or the Mother of All American Family Feuds (Civil War). Oh, you mean practicing in front of a mirror. I use to at home whenever I got a new holster. Practiced the draw 1k times to get it smooth and then fast.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!