MBS Rating (lbs): 2,810 MBS Rating (kN): 12.5 Ideal as a back-up while on long rappels, we constructed our AutoBlock from a durable 6 mm cord sewn into a 13.5-inch loop. The AutoBlock attaches from the harness to the brake-hand end of the rappel rope just below the rappel device and works as a prusik or Klemheist knot.