Schematic Diagram - Variable Vane Operation

A = Low engine speed: B = Intermediate engine speed: C = Maximum engine speed. 

Low Engine Speed.  At low engine speeds the mass flow of exhaust gas is low so the vanes are moved towards the closed position to reduce the Variable Geometry Turbine (VGT) inlet area. This reduction causes an increase in the gas velocity into the turbine wheel thereby increasing wheel speed and charge air pressure.

Intermediate Engine Speed.  As the engine speed and the mass flow of exhaust gas increases the vanes are moved towards the open position to modulate the boost pressure.

Maximum Engine Speed.  At maximum engine speed the vanes are almost fully open to maintain the gas velocity into the turbine wheel. The maximum position of the vanes is also an emergency default position in the event of an electrical fault. This lowers the chance of engine damage due to excessive charge air pressure.

When the vehicle is driven at high altitudes, the ambient pressure reduces causing the compressor wheel to rotate faster to achieve the same charge air pressure. To prevent the turbine wheel from over-speeding under these conditions, the Powertrain Control Module (PCM) protects the VGT by opening the vanes further. This reduces the turbine wheel speed. This is known as the altitude margin of the VGT. The PCM uses an internal barometric pressure sensor to monitor altitude.