Can anyone explain BOV spring ratings to me?

The Blow off spring is rated at a certain pressure in your example, it is 11psi. There is an adjustment screw on the top that will most likely get you another 10-15 lbs above the springs rating. Turn it clockwise to compress the spring which will put more pressure on the valve and keep it closed longer. When you let off Vacuum will pull the valve open which will relieve the pressure to the turbo's

When mine was on the dyno a few weeks ago, at idle both blow off valves were open. I was pulling about 22psi vacuum. I think I have 12lb springs in them. So I just loosened the jam nuts on the adjuster's and turned them 3/4 of the way in which kept them shut. If the throttle was wrapped on and then shut, the spike in vacuum went above 22psi for a brief moment which pulled both valves open instantly and then they would close immediately afterward.