The exhaust camshaft is timed with the intake camshaft and crankshaft to push open exhaust valves, allowing exhaust gasses to escape through the exhaust system after the combustion process takes place. The lobes on the camshaft are what push the exhaust valves open with use of the rocker arms. These cam lobes can wear down over time, either from excessive stress or contaminates in the oil, which doesn't allow the camshaft to push the exhaust valves open completely, causing exhaust gasses to remain stuck in the combustion camber.