In September of 2003, with an eye on further ensuring the safety of competitors, NASCAR announced that racing back to the caution was no longer permitted in all three of the sport's national series. That rule change led NASCAR to institute a procedure where the field is "frozen" on the track once the caution comes out.
So what exactly does "freezing the field" entail?
The cars' positions on the track are determined by the previous timing-and-scoring line that the cars passed on the track. As the diagram illustrates, there are timing and scoring lines all over the track (A-H).
In the diagram above, cars 1-3 (are on the backstretch) and are scored by their running order when they passed the timing and scoring line E. Cars 4 and 5 are scored by the positions they held when they passed timing and scoring line D. Cars 6 and 7 are scored by their positions when they passed timing and scoring line C.
During the time that a field is frozen, pit lane remains active with pit-road speed in effect. The cars that are pitted from the pit-road entrance to the start/finish line, Cars 11, 12, 13, and 14 in the diagram, must reach the start/finish scoring line A that extends across pit road before the race leader (the No. 1 red car that is approaching scoring line A) reaches the same line on the race track. The cars on pit road that reach that point first will not lose a lap to the leader. If the leader reaches scoring line A before cars 11-14, then those cars would lose a lap to the leader.
Cars 8, 9, and 10, which are pitted from the start/finish line to the pit-road exit, must reach the pit-road exit scoring line before the leader (the No. 1 red car that is approaching scoring line B) to avoid falling a lap behind. In the diagram above, car 8 would not lose a lap, while cars 9 and 10 would.
If a driver attempts to speed in pit lane to avoid going down a lap to the leader, that driver loses a lap and is moved to the tail end of the longest line.
If, when a caution comes out, the race leader does not slow immediately in an effort to put pitted cars down a lap, the leader will be hit with a penalty by being sent to the tail end of the longest line and all cars that have pitted will retain their lap positions.