In yellow flag situations, you will see the drivers wiggle the cars from left to right, to get rid of debris building up on the tires that are cooling down. Most of the debris is small pieces of rubber that are broken off the tires of the cars while driving around the track, these small pieces are known in the racing world as marbles. The reason it's called marbles is that it causes the cars to lose traction as if they are driving over real marbles and in safety car formation many of the drivers are forced to drive on the lines where the debris is laying.