A phrase used in NASCAR
to describe a large crash at Daytona, or Talledega involving several cars. Usually caused by the restrictor plates on the cars keeping them bunched together, 3 or 4 across, 43 deep, untill one car in the front cuts down in front of another car to close, and spins out in front of the rest of the pack. The Big One usually has 5-10 cars involved but some have had up to 26.
Just look up "The Big One @ Daytona" and you will get a pretty good idea.