Mark is leaning forward with great interest. On the screen the fraying cable becomes magnified and rocks back and forth over a pulley. All mechanical things wear and break from use. Our muscles and nerves are no different. Overuse without allowing the body time to repair, causes muscles to thin or to tunnel.
Mental stress causes muscles to stay tight, or spasmed, causing the same damage.
Even nerve fibers can fray from too much flexing in one position. They can become raw and irritated, or actually break strands. The cable frays apart and the block falls with a resounding thud. Kenrick notes Mark's reaction and the other's intent interest. He turns on the lights and turns off the projector.