There are 2 primary ways low flow shower heads restrict water flow. The first is to create a pulse. With this method water flows for a fraction of a second, is cut off for a fraction of a second flows, is cut off, flows, is cut off etc. This way only half the total amount of water is used. The other method is to introduce air into the water stream. This produces a physically lighter flow as some of the space within the stream is taken up by the air.\r