I have this! After a month of having Casper, I'd never seen him flare up. I thought this could boost his confidence a little bit (a threatening male would retreat (I'd take out the mirror), and he'd feel accomplished). I tried it for the first time a week ago, and he immediately hid in his cave. Poor thing. I took it out that very second. I tried it again today, and he was actually interested. Sure enough, he flared! He was very lackadaisical about it, and in less than a minute, he wondered off.
Sometime soon, I'll try having him in a large tank with some tank mates. I'll use it very sparingly, so as not to make him aggressive.