One can concentrate in any of the three centres which is easiest to the sadhak or gives most result. The power of the concentration in the heart-centre is to open that centre and by the power of aspiration, love, bhakti, surrender remove the veil which covers and conceals the soul and bring forward the soul or psychic being to govern the mind, life and body and turn and open them all fully to the Divine, removing all that is opposed to that turning and opening.
     This is what is called in this yoga the psychic transformation. The power of concentration above the head is to bring peace, silence, liberation from the body sense, the identification with mind and life and open the way for the lower (mental, vital, physical) consciousness to rise up to meet the higher consciousness above and for the powers of the higher (spiritual nature) consciousness to descend into mind, life and body.This is what is called in this yoga the spiritual transformation. If one begins with this movement then the Power from above has in its descent to open all the centres (including the lowest centre) and to bring out the psychic being; for until that is done there is likely to be much difficulty and struggle of the lower consciousness obstructing, mixing with or even refusing the Divine Action from above. If the psychic being is once active this struggle and these difficulties can be greatly minimised.
     The power of concentration in the eyebrows is to open the centre there, liberate the inner mind and vision and the inner or yogic consciousness and its experiences and powers. From here also one can open upwards and act also in the lower centres; but the danger of this process is that one may get shut up in one's mental spiritual formations and not come out of them into the free and integral spiritual experience and knowledge and integral change of the being and nature.

                                                   Sri Aurobindo