Manual Valve
WARNING: This page is about a different car, the 2008 Pontiac G5 and 2008 Chevrolet Cobalt. 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.