Experimental investigations have shown that the flow discharged from a centrifugal compressor or pump impeller into a diffuser is often unsteady and axially asymmetric. A theory based upon a simplified model is developed for this type of flow. The theory predicts a significant reversible work transfer between parts of the flow with different velocities. This effect only occurs in cases of rotating distortion patterns; stationary distortion patterns, therefore, behave differently. Theoretical results are compared to measurements and good agreement is found. The implications of the theory have been investigated by means of machine computations and are discussed herein.