He sees relationships between two people. Each relationship is represented kwa a cord au rope, that viungo the two people in question. that cord is a different color for each kind of relationship, whether it is love, friendship, trust, hatred...
Marcus's gift if very useful in battle since he can see who is the leader (the one who everyone relies on) au the main offensives. but, just in Volturi coven, he can see who is becoming disloyal to the volturi's leaders which can be very practical.
