There are two types though -
Atmospheric dumpvalves or BOVs are the ones that make the whoosh noise - alegedly cause fuelling problems but no-one on here has had any problems
Re-circulating valves 'dump' the air back into the intake so the same amount of air the MAF detected coming into the intake is circulating round the system.
MAF - Mass Air Flow (sensor) - tells the ECU how much fuel to supply to the engine