What is a Gicleur / Gigleur / Giggler / Flow Restrictor?
A properly designed espresso machine has a restricting aperture inserted into the path of the pressurized water to control the amount and rate of its flow. The technical name of that device is "gicleur." Although it is often referred to as the "gicleur valve," it is not a valve at all; instead it is a limiting aperture, normally between 0.6 mm and 0.8 mm in diameter.
The gicleur plays several important roles in the functioning of a properly tuned espresso machine. First, it limits the amount of water that flows into the portafilter in a given period of time. Slowing down the flow of water is important for the production of quality espresso.
Second, it protects the coffee in the portafilter from being blasted by the initial pressure wave that emanates from the pump as soon as it is turned on. This pressure wave travels down the tubes and impinges on the top of the coffee in the portafilter if the restrictor is not in place to dissipate this wave. In the absence of the gicleur, the initial blast of water creates a crater on the top of the coffee puck, thereby making the center portion of the puck much thinner than the surrounding portions. This causes most of the water to channel through this thin coffee in the middle of the puck. When that happens, the resulting liquid will be weak and bitter because the center portion of the puck would have been over-extracted and the surrounding portions of the puck left under-extracted.