It's a gyro system, meaning that the rider can do "Bar Spins" without getting the rear brake cable tangled. They are standard on many BMX bikes, can be a little fiddly to set up particularly on lower end models. Many riders these days either ride brakeless or with only a rear brake but with a very long direct cable so that bar spins can still be performed.
http://en.wikipedia.org/wiki/Detangler