Since “Mystical Space Typhoon” cannot destroy itself, when you chain “Fiend’s Hand Mirror” to your opponent’s “Mystical Space Typhoon”, you cannot re-direct the “Mystical Space Typhoon” to target itself. But you can re-direct it to target and destroy “Fiend’s Hand Mirror”.
When your opponent activates “Mystical Space Typhoon” to destroy his/her “Mirage of Nightmare”, you can chain “Fiend’s Hand Mirror” and have “Mystical Space Typhoon” destroy “Fiend’s Hand Mirror” instead.