Generally, Turbocharger is an air pump. Turbine wheel is rotated by the blow of hot exhaust gases which leaves the engine after combustion. When the turbine wheel rotates faster and faster, the compressor wheel also spins quicker while both the compressor wheel is connected to the turbine wheel through the shaft. The high speed rotation of the compressor wheel also pulls and compresses more air inside before drawing it into the pumping it into the engine's chambers.

In order to response to the change of power output due to the throttle change relative to a naturally aspirated engine, the time is needed, and it leads to a hesitation or slowed throttle response accelerating, it is known as turbocharger lag (turbo lag).  It is because the required boost by exhaust system and turbocharger need time to generate it due to the Inertia, friction, and compressor load. However, by improving technology, supercharger do not face this problem while the compressor is directly powered by the engine without the need of turbine.

Turbocharger can be applied into different applications, here divide into two areas, the one does not require the changes in output power (like marine, aircraft, commercial automotive, industrial, engine-generators, and locomotives) and the other requires. For those applications require rapid changes in power output such as automotive, the turbo lag will be a significant problem.
 

The nozzle vane “closes the vane” in order to respond to low speed by increasing the gas speed through the reduction the nozzle area into turbo and make the air more powerful to improve the low engine speed. In contrast, when the speed of engine increase, the actuator make the nozzle canes to open position to allow the maximize exhaust flows into.
The variable nozzle (variable geometry) of turbocharger is designed to the needs of needing to balance the high speed efficiency and low speed response, by matching the expected boost of the engine through the changing of the exhaust gas inlet area with the engine speed.