Go look at the besieged region info quick and put the cursor over one of the mob strongholds. See where it say Enemy forces? When that reaches 100 the mobs beging to prepare for an attack, depending what level they are coming as once it reaches that level they start walking to whitegate(ie for a level 8 once it reaches 190-200 or 200-210 i forget, they attack). When you turn in that mirror it will lower there number by 1.
You cant lower the number once it goes over 100 so you have to use it before they start preparing. Not really to useful of an item, since people still like to gain the easy ISP from besieged I think, I guess its almost a negative. Hope this helped.