The provent housing has a valve in the cap which pops open and vents to atmosphere if the pressure on the crankcase/provent inlet gets too high.
So in your example, if the engine blows a ring, the flow rate of gas into the crankcase increases, as does the pressure in the crankcase and the pressure drop across the filter. The valve then opens, and the result is you have an oily engine bay, but the motor is fine as none of the oil gets into the intake.
(as someone else mentioned, there is a one-way valve for the drain, or you can run it into a catch can and manually return it if worried).
It really is a pretty well designed bit of kit.
If you need further convincing I can take a pic of the valve 
 
  
The filter media in the provents would be more efficient than a K**&**N if you used it as an air filter...
Bookmarks