Why should not be behind the sofa with mirror back

Someone to make it seems more fully capacious sitting room, will be on the wall to hang a mirror.Because there are actually, in fengshui, mirror reflection effect, therefore not hang at random.Sofa should not be a large mirror, behind a person sitting on the sofa, people can clearly see from the mirror of the sit back, and then greatly, which leads to time-consuming restless spirit.And if it is a mirror in the side not later, back won't be reflected from the mirror, it might just as well.