Can anyone explain BOV spring ratings to me?

Ok so best I can figure it in a twin Turbo blow through system that uses a single fmic only one BOV is necessary as it is plumbed into the intake pipe before the plenum and after the fmic, that being said most of the BOVs I have seen the springs are rated at 11psi or less. I'm not planning on running much more but I'm wondering if I'm interpreting this rating wrong? Will this cap the system at the spring rating or just begin to relieve pressure? Will it only actuate when the throttle is closed and there is that amount of pressure? Is it a way of measuring vacuum? I understand what the BOV does I'm just confused as how the spring is used to accomplish it. Corky and Google have been no help....