Supposedly, cars coming out from the manufacturer aren't grounded properly so that electricity needed for various systems is lost. By attaching a thick wire from the block, tranny and other major components to the frame, the car is better grounded. The company behind the Hyper kit claims that this will add about 5 hp and 5 lb/ft of torque generally across your powerband (dyno-proven).

I'm still skeptical about how this actually equates to power gains but your interior lights will get brighter, as well as your headlights and you'll reduce that dimming that happens when you turn on your stereo, power windows, etc.