According to the description, it seems that the attack will be reflected back with 10x original damage. This time I encountered an AI Superfortress -- with its 1600x250 attack and 1,500,000 health, I threw full caps of MkI and II mirrors at it, expecting to reflect back 16000x192 damage (or at least, 1600x192 in case I understand incorrectly). If it is the former, the SF will be killed right there, and if latter, have a large portion of its health chewed through.
It turns out that my entire loss of Mirrors (MkI one shot, MkII two shots) only dealt less than (approximately, since I am spamming youngslings too) 1% of SF's health, and gave me a reprisal level 2. Really?? I thought it just happens on guard posts. That basically rendered Mirrors useless isn't it? With its 10x damage to self, slow speed, and low attack.
Now, in this game I wanted to test the ability of several interesting ship type, so Mirror was actually added by editing save file. I know that some ship types (e.g. Protector Starship) cannot be added this way, is there anything special about Mirrors so that they just behave differently because I manually added them?