The 6.7L VGT actuator uses engine oil pressure on a piston to move the vanes on the exhaust turbine.
The VGT actuator is commanded by the PCM, based on a desired MAP pressure. This actuator meters engine oil to either side of the piston.
This design feature reacts quickly to changes in demand based on driving conditions. When one side of the piston is pressurized, the opposite side is vented.