The emergency brake is the key component in powersliding. By Slamming the E-brake, you can cause your rear tires to lose traction and throw your car around a tight turn hellaciously fast. It takes a long time to learn, and only works well with powerful rear wheel drive cars, but is REALLY cool to watch, and can shave seconds off of a lap. I cant do it, but a friend of mine can. He can tear up SS11 in GT2 in about 15 seconds less than I can do it in.
I sometimes hit it to try to slide sideways into a wall instead of going head first and having to back out. (last resort)