The plates don't stay "mostly open" (I don't think) but they definitely don't close all the way. Based on my experience with my car, that's how the engine handles IAC, instead of an adjustable bleed valve or whatever. I don't see why a roots style blower would change that much, plus it would have to have a centripetal clutch anyway (like basically every factory supercharged setup does) to keep idle quality and whatnot.
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