The impeller is using centrifugal force to throw the refrigerant out at high velocity. The diffuser then converts that energy into a static pressure that is read in the condenser and discharge. The Variable Geometry Diffuser (VGD) is a steel ring that slides out into the diffusion channel. This ring helps convert more of the energy to pressure in order to better manage high lift. The VGD ring is driven by an electric actuator on the outside of the compressor housing. It operates the same as the PRV with a floating point style of control.