A concave bagua mirror is used in feng shui applications when any negative energy outside your home needs to be neutralised by being absorbed. Basically, the concave design of a mirror will "suck in" the negative feng shui energy and neutralise it.
The first feng shui rule for using the bagua mirror is that it should never, ever be used indoors.
The second feng shui rule for using the bagua mirror is that you use it only if there is bad feng shui energy - Sha Chi or Si Chi - facing your home or business. The feng shui bagua mirror is not a decoration feng shui item and should not be used as such.
If you do find yourself in a position that requires subtle protection of your home or office from potentially dangerous Sha Chi feng shui energies, (sharp or attacking structures pointing at your home), or bad energy coming from your neighbours, you may choose to use the bagua mirror.
Best is to place a feng shui bagua mirror when advised by a professional feng shui consultant. Traditionally, a feng shui bagua mirror is placed above the front door or above a big window that is facing questionable outside energies.
Using outdoors: Please hang your bagua mirror as much as possible in a protected place, to shield it agains atmospheric influences.