Excess "air" pressure that builds up under the valve cover is vented to the air cleaner. If the pressures are sufficient, it will also push valvetrain oil thru the system, although there is usually an oil baffle that prevents most of it from getting to the air cleaner. An excessive amount of oil resulting from a lot of air pressure build up could be a sign that there is a compression leak, either from the valve seals or the valves themselves, allowing combustion chamber pressurization to leak out into the camshaft/valvetrain area under the valve cover.