Within the engine crankcase (not the cylinders) there is air,oil and maybe a little blowby from the burning of gas. So as not to polute the atmosphere this can be retuned to be burned A small screen looking like a thimble with holes (a strainer) is placed in front of the throttle plate. Should this get clogged, pressure build up in the crankcase will cause the gases,oil to seek another way out (rear main seal) to releive the pressure.Should you also have a backfire the screen should prevent an internal burning of any unburned fumes.A PCV (if it had one)would return the same gases but would be located in the intake manifold.