Tumbler Generator Valves (or "TGV" as they are most commonly referred to) can be found on Impreza WRX, Forester Turbo and Legacy GT models from 2001 onward.
Their prime purpose is to reduce emissions from cold idle to meet standards governed by the EU emissions regulations (similar regulations also exist for both the US and Japan). Whilst this may be good for the environment it does little to aid the performance of your car and it's not uncommon for the sensors and motors connected to the TGV body to fail (leading to "limp mode", CEL lights and ultimately considerable expense in replacement parts and code clearing).
The standard TGV's are similar to the design of your throttle body, encompassing twin "butterfly" valves in each tumbler housing to control air flow. Even when fully open these restrict the flow of air traveling into the cylinder head ports, reducing performance. This coupled with the possibility of sensor and motor failure mean there is good call for an improved alternative!
RCM Billet TGV Delete's do not contain any butterfly valves, sensors or motors and allow for a smooth, direct flow of air into the cylinder heads. Whilst it is necessary to initially adjust the ECU mapping to prevent a check engine light the results offer; increased performance, improved reliability and a much improved esthetic appearance (thanks to our crinkle black or red coated finish). The assemblies are fitted with O rings, removing the need for gaskets between them and the manifold. The kit comes complete with the gaskets between engine block to lower TGV and replacement bolts to secure both the manifold and tumbler castings to the engine block.