The diffuser screen (or shower/dispersion screen) distributes water across the coffee puck evenly. It is important to clean it regularly and to inspect it for wear. The screen is subject to the pressures of the pump, abrasion from the coffee and potential physical damage from the portafilter. It can also be damaged if not removed before changing the group gasket. The KB90 screen has shorter sides than others due to having to allow room for the piston to move.
