Turbochargers works on the simple principle of increasing the intake air density by compression. Being able to fill more air into the combustion chamber will allow more fuel to be added to produce more power.
However the operation of the turbocharger relies solely on the exhaust gas velocity to drive the compressor. Thus the compressor will be at optimum operation range when the engine is under heavy load. When the throttle is opened, it will take a certain period of time (depending on the turbo's characteristic) for the turbine to spin up to it optimum rpm and the engine intake does not see as high a pressure as it would want thus this delay in the power delivery is known as "lag".
To reduce lag, one of the direct way is to simply use a smaller turbocharger as the turbines are smaller and lighter, it has lesser inertia and thus takes a shorter time to spool up. However due to the smaller size, the volume of air that it can take in for compression is restricted at high end and thus the performance is choked. Vice-versa to allow more air to be compressed and thus more power to be produced a bigger turbocharger is normally chosen. However this will directly increase the turbo lag. Thus its often a dilemma for engine builders.
This lag is not desirable as it creates an suddenly surge of power after a period of lower power... thus this non linear power delivery will affect how the vehicle have to be driven/ridden.
It is this problem that led to the development of the Variable Geometry Turbocharger. It is simply a turbocharger with an additional set of fins which regulates the flow of exhaust gas into the turbine. This additional set of fins are controlled by an electronic actuator. The opening and closing of the actuator varies according to the engine's load (ie. rpm).
At low rpms, the fins are closed to a small gap allowing the slower exhaust gases to accelerated into the turbine region (theory based on fluid...