The flow restrictor is placed inline prior to the grease trap. Its purpose, by design, is to slow the flow of water into the grease trap so that it is not overwhelmed, and to allow air into the trap to assist proper flow. The restrictor has a small hole inside (larger as the trap size increases) that can easily become blocked with food solids or hardened grease. The restrictor is a serviceable component and MUST BE PLUMBED SO THAT IT CAN BE SERVICED, usually with rubber connectors. Another problem that occurs over time is the air valve part of the restrictor can clog with accumulated grease caused by the natural swirl of water through the pipe. the only way to clean this is to be able to get to the air valve. Some restrictors, by design, are more prone to problems than others. Look for a restrictor that has a large air vent, equal to the drain pipe size or atleast an inch and a half. Restrictors with a half inch copper air valve is just a problem waiting to happen! In addition, it is better if the actual restrictor (the wall inside that contains the hole) is solid metal, not plastic.