[andertusa]

Yes that is true, the design of the system is that it gently sucks the vapors out of the crankcase and runs it through the cylinders to be sent out as exhaust. I personally have never been a fan, I built an engine for a friend and used 2 breathers (one on each cover) on it and he hasn't had problems. I built one for me with 4 breathers (2 each rocker cover), no problems after 28,000 miles.
Of course there are also vacuum pumps to create an internal vacuum in the crankcase that helps develop more power because of less blowby and allowing the engine to have looser tollerance rings.