This Key can be summed up in one word with two expressions - REFLECTION. If the Fool represents inspiration, the Magus clothed it in language giving it direction, the Priestess is substance and the medium for the word to be received and cast forth. By her very nature, she is duplicitous: she is dark yet brilliant in the light of the sun, rhythmic and cyclical yet stable. She securely rests on her throne, poised between pure being and the matrix of the human personality, reflecting what is cast before her.
We find her highest expression in those moments when the light of the Sun of one's Soul is most clearly reflected outward, through the personality and into the world in the same way the Moon reflects the most of the Sun's light during a "Full Moon" (which, even then, is an illusion, since we are not necessarily seeing the moon in its ENTIRETY but rather the a face of the moon).
We find her lowest expression in those times when we are enmeshed in the matrix of the personality and our view of ourselves and the situations we find ourselves in is skewed.
Interestingly enough, the act of reflection can loosen the grip of that which we got ensnared by, giving us a chance to discern the illusions as such and return to that state between pure being and the personality. It should be noted that what is said in the prior sentence is crude and general statement summarizing the mystical journey.