Notice this is coming from fresh air between the MAF and the throttle body. This clean air gets to travel through the heads oil passages, down the oil return galleys of the block, and into the oil pan crankcase area. Along the way it will get mixed with fumes, mists, and condensate. The closed vacuum side that in some cases includes a catch can, then pull or vacuum suck the now dirty mix toward the intake for recycling. Hopefully our catch cans will get most of this.