Thats the blow off valve everyone talks about. See the list if you want to know about its noises or that it does/doesn't cause boost creep/leak. The top small nipple is the control nipple... when about 4" of vac is applied, the diaphram opens allowing air to pass from the side nipple through to the bottom nipple. The long hose that curves out the bottom goes to the bottom of the thick intercooler line that runs next to the water box. the other side goes to the huge airbox line.