Larger turbos have trouble drawing in air (imagine the energy you would need to draw positive suction on say, a wheely bin, it would be immense) so around the outside of the turbo (or standard rotary supercharger) is an air diffuser ring. This is a plate that has a divergent nozzle effect, there are a large number of these nozzles arranged all the way roundand due to the effect of the rotary compressor, each of these nozzles create a low pressure air cyclone which helps to pull in more air to the compressor, raising the boost pressure and increasing mass flow rate. The air diffuser plate is really simple, it is basically a flat plate with what look like match sticks on it, wide open at one end and close together at the other (hence nozzle).
Variable nozzles simply move in and out to vary the cyclone (on larger units anyway)
