Manual Valve
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
Mechanically moved through the gear selection, the manual valve allows line pressure to enter the D321 fluid circuit, and the PRDN4 fluid exhausts at the valve. D321 fluid is then routed to the spring end of the 3-4 shift valve.