Manual Valve
WARNING: This page is about a different car, the 2004 Pontiac Sunfire, 2004 Chevrolet Classic, and 2004 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
With the manual valve in the reverse position, line pressure is directed into the reverse fluid circuit, in addition to the PRND4 and the PRN fluid circuits, which were used in Park position.