When the parts of a paved race circuit outside of the driving line become covered in rubber worn off from the racer's tires. This condition makes passing more difficult as the pieces of rubber reduce traction off the racing line. During their cool down lap, drivers may intentionally pass over these areas to add weight to their cars for post-race inspections that require a minimum weight. First heard from David Hobbs, NBCSports Formula 1 Commentator.
The 2013 Formula 1 Chinese Grand Prix became a klagmire when the soft compound Pirelli tires degraded in less than 10 laps.
by Air Cav Hokie April 19, 2013