I guess the simplest way to think of it is like this, a vacuum or suction is created at the collector, by the exhaust gasses passing the fitting welded into the collector. This "suction" is then routed to the valve covers via the hose. This vacuum works to reduce any positive pressure which may build up inside the crankcase cavity of the engine. It's supposed to create a better ring to cylinder wall seal, and has been said to reduce oil leaks as well.

I'm sure someone else can explain it better.

codfish