How? Exhaust is a function of the flow of the exhaust side of the engine while the valve is open. The exhaust gets some assist from the intake which will let in cool fuel vapor to displace the exhaust gas during the valve overlap period. Little to no compression happening at this time.
Race motors typically have a lot overlap in which both intake and exhaust valves are open. This allows the heavier, cooler intake charge to displace the less dense, hotter exhaust gas in the cylinder. The open exhaust valve and downstream passages allow the exhaust gas to escape and evacuate the cylinder. The engine builder will specify or select a camshaft that will allow the engine to produce optimal power within the rpm range required by the application with an eye also on how long the engine has to live.