VNT (Variable Nozzle Turbine) or VGT (Variable Geometry Turbocharger) turbos use movable vanes that change the angle at which exhaust gas hits the turbine.
At low RPM the vanes close to speed up the gas and cut turbo lag; at high RPM they open to prevent overboost. This gives balanced performance in town and on the motorway.
The most common issue is vanes seizing from soot build-up. Regular cleaning and correct actuator calibration solve it. In our workshop we clean and calibrate VNT/VGT vanes with test equipment.