What does a crankcase evac system do?

Can anyone explain to me the science behind a crankcase evac system. The one where you weld the bung into the header collector and then run a tube off of the valve cover breather on each side.

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

Yes.
And getting that tube positioned properly, with the cut end facing correctly in the collector is important for best operation.

Should I even consider it for a 12.00 second car or is it not really necessary? I was just thinking that since I am going to be switching over to 1 7/8" headers and they are used, I could weld any fittings, bungs, etc. onto/into them before I get them coated. Just wondering if the evac system is worth it or not on my car?

I assume that if you are using an AFR meter you would need to weld the evac connections in AFTER the O2 sensor bung...right?