A picture is worth a thousand words, right? The curves on this graph represent the action of an intake and exhaust lobe, with lift indicated on the vertical axis and degrees of duration across the bottom. Note the red exhaust-valve curve. It opens before bottom dead center (BDC) and closes after top dead center (TDC). The intake valve, represented in blue, opens before TDC and closes after TDC. Right smack in the middle, where the two curves intersect, is the overlap area. This chart also points out the intake and exhaust centerlines, as well as the lobe-separation angle. The 360 degrees represented here denote one rotation of the crankshaft, but remember, two rotations (720 degrees) are required to complete a four-stroke cycle.