As the vanes close they constrict the flow accelerating it at an angle to the turbine blades thus causing the turbine to spin faster at the expense of more back pressure. As the vanes open up they drop back pressure but also reduce the turbine speed.

In general you want maximum boost across the widest rpm range possible ( neglecting emissions EGR flow considerations ). Ideally you want a system that opens the vanes at rpms to low to generate boost even with a VGT and closes them when boost can start to be created and opens them up to regulate maximum boost.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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