"The logic can be seen by analogy to quality control along an assembly line. A polymerase is like a machine that makes widgets and sends them down the line at a certain rate. An exonuclease is like a worker responsible for removing defective widgets that come down the line. The worker sometimes removes perfect widgets by mistake. (The fewer such mistakes, the more "specific" the worker.) However, the number of defective widgets that get past the worker depends primarily on how many widgets the worker checks as the assembly line rolls by. If the assembly line slows down (i.e., there arises an impediment to forward translocation), the worker will be able to check more widgets and therefore let fewer defective ones go by. "