The shield means protection.

There has been progress in all these parts, but they seem to be subject still to a response to the suggestions of the hostile forces. Everybody gets these suggestions, but they ought not to be allowed to enter inside, especially in the heart, or to be accepted by the vital. Evidently, they enter through the physical mind (from the throat upwards meant that) and effect the surface vital and emotional being. You must get the power to reject them from there by a constant and steady denial and refusal of their suggestions. So long as anything in you says "yes" or accepts, there is always the possibility of a return.

If the attacks of the hostile forces have been made less strong by concentrating in the heart (or if they have become less frequent) you must continue that concentration until you are able to join the head and the heart, the psychic and the higher consciousness. It all depends on that. The psychic must be strong enough to compel the vital and physical to give themselves to the Divine or the higher consciousness must so descend and occupy everything that the old movements can only at most move on the surface without being able to enter in or touch the inner calm or the two together, psychic and higher consciousness must occupy the whole being. These are the three ways in which yoga moves. If the concentration in the heart which means the awakening of the psychic is most effective against the attacks, then it is that you must follow.