The diffuser is the rear white plastic part of the Waterway jet. Make sure you have identified how the diffuser locates onto the jet - it will only clip into one position. Important: Regarding the Powerstorm diffusers, depending how old your jet is you may find that the lip on your existing jet face is to large to fit into the slot on the newly designed diffuser and therefore you may have to carefully cut down this lip to fit or alternatively replace the complete jet.