This kit is used to control converter exhaust pressure in order to loosen the converter when needed. This external dump for the TH400 also includes instructions and parts to modify the transmission in order to make the dump effective. The single internal dump valve will typically raise stall speed by 500-800 RPM. This dump valve can either by controlled by the Hughes controller linked below, or many popular aftermarket ECU's.
Here is a great video with some more information on this product: