A turbocharger is a turbine and compresses incoming air into the engine and is driven by exhaust gases. There are many advantages that turbochargers bring; however, one of their main drawbacks, besides cost is their fixed geometry.
A variable geometry turbocharger is a turbocharger whose geometry and thus effective A/R can be altered as needed while in use. There are several types, but the most common design includes several adjustable vanes around a central turbine. Thus, the variable geometry use vanes to change the air flow path of the exhaust gases which maximizes the boost across the entire rev range. The angle air of air flow onto the turbine blades changes as the angle of the vanes change and because of this the aspect ratio A/R changes as well.
Check it out!