When you're driving a
race car you want to get as far in front as you can, to make it hard on your pursuers. The problem with that is such races really aren't very dramatic, or at least the drama may be pretty far back in the pack, something which disinterests most casual fans. People want to see a shoot-out for the win. A
NASCAR Yellow is racing slang for a
full-course caution thrown specifically to gather up the field at the end of the race to ensure a dramatic finish.
Why does a NASCAR Yellow work? In road racing, many or most incidents are handled under a local yellow, meaning a yellow flag is shown at a particular corner and racing is unrestricted once drivers pass the incident. Normally this means no passing until both cars have passed the incident in question, such as car into the wall. In other words, the caution only covers a small portion of the course. Road racing began on closed public roads, often before good radios were available, and many courses were very long. For example Spa-Francorchamps originally was over eight miles long, LeMans longer. In the old days, on such a long course, local yellows were the only practical way to run a race. Road racers got used to working under a local yellow (caution).
In oval racing there is only one type of caution, the full-course yellow. Oval tracks started at the county fairground. Ovals are almost always much shorter then road courses. At 2.5 miles Indianapolis is a very big oval--compare that to the Nurburgring at 14 miles. I have raced at ovals short as 1/5 mile while the shortest road course I know of is 1 3/4 miles. This means an incident in one part of the track affects everyone, a factor made worse by the very high speeds attained on high-banked super-speedways like Daytona and Talladega. Shorter tracks and the oval layout makes it much easier for the stewards to put out a yellow everywhere at once. So they just throw a full-course yellow and be done with it! Even on a road course, no NASCAR sanctioned race or series (such as the Grand AM) will extract a car under a local yellow. If a car must be moved, they always throw a full course yellow. Road racers also use full-course yellows now that radios other forms of communication are available, though we usually reserve them for incidents deemed too serious to be handled under a local yellow. If a guy gets a big lead, he gets to keep it.
Under a full-course yellow, the pace car gathers up the field and enforces a slow pace. This protects race workers charged with clearing the course and forces the cars to bunch up. Racers end up right behind the car they were chasing. This bunching of the field keeps more cars on the same lap and generally tightens up the racing. More then any other sanctioning body, NASCAR realizes its income depends upon 'the show' and has been known to take dramatic steps in order to equalize the competition, thus raising the drama. Although most yellow flags thrown on a NASCAR circuit are legitimate (that car in the wall is a dead giveaway) many in the racing community very strongly suspect that NASCAR stewards regularly throw a yellow flag for little or no reason when they think the competition needs tightened up.
When a caution is thrown for little or no apparent reason racers call it a NASCAR Yellow. Debris on the course (which if genuine can cut a tire or cause other major hazards) is the usual reason given for such yellows. The field tightens up, no one gets to far ahead, so there is always a race to the checkered flag. The fans enjoy the drama, the sponsors enjoy the ratings and NASCAR reaps the profits, so a NASCAR yellow may be considered a win-win situation for everyone except racing purists and the driver whose big lead just got wiped out.