How a mirror may be used in training:
Stare into the bathroom mirror and practice saying "Are you talkin' to me?" This requires a variety of intimidating facial expressions to match the voice inflections. (Taxi Driver- Robert DeNiro)
Taking a hand mirror, holding it in front of me and looking downrange behind to aim at the target. (Buffalo Bill and Annie Oakley trick shots)
Seriously, though I have used the mirror to practice quick draw techniques.
NRA Safety Range Officer