A sexual act, and a certain form of bukkake. A white woman has a group of ten African-American men or more, completely naked and ready to screw. One man jerks off over the woman at a time, and he only has 30 seconds to ejaculate. The next man in line repeats the process, and so on, and so on, until the woman is completely covered with semen.
The origin of this term is quite simple, but a little racially biased. Downtown, referring to the African-Americans, symbolizes "gangsta," "pimp," etc.
Derby, however, is a different story. Racecar driving, AKA "NASCAR", is only watched by Caucasian rednecks, which is why it refers to the white woman. It also signifies why the males have to ejaculate so quickly.

Your a Grillbilly when your grill is the nicest thing about your house.