What is the importance of the "string snare" in the novel, Child 44?

A string snare is used by Leo/Pavel and Andrei as children to catch animals, such as cats, to eat. String snares are later found tied around the ankles of murdered children, and have no seemingly apparent purpose to Leo. Leo only later discovers the snares are among the clues that Andrei uses to sign his victims, in order to get Leo to recognize Andrei as the killer, and seek him out to reunite with him.