In NASCAR, "bleeding tires" refers to a process of adjusting the air pressure within a race cars tires. This is done to prevent the tire build, or increase in pressure, from getting too large by releasing some of the pressure once the tire pressure reaches a certain level. Bleeding tires is an advantage for drivers, and there have been many ingenious methods developed over the years to do it. However, the use of bleeder valves, which release air from tires as they heat up, is prohibited in NASCAR. NASCAR penalizes drivers who use illegal methods to manipulate tire pressure, such as using special valve stem caps to cheat the rules.