Crankcase breather question

Ok guys forgive me but i still am confusing myself, ive reread the thread about 3 times. Im still having a hard time understanding the transitional value for the pcv vacuum. So for example if i had 10-12 inhg of vacuum at idle and had a pcv with a transition in the 8 inhg range wouldnt the pcv valve not open? I know its supposed to be less than idle vacuum but how is it only partially open at idle with the max vacuum level the engine has but open more with less vacuum? Is the valve in its farthest upward position with vacuum applied considered closed? Also where is the normally closed or partially closed position of the pcv? Because ive always though where the valve is sitting with no vacuum applied is considered closed. Or is this position just a check valve to close it off when the engine is off and prevents vapor from going in the opposite direction? Be easy on me lol i have a headache from reading