1-2 Shift Solenoid (SS) 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.
The 1-2 SS valve de-energizes (turns OFF), allowing the 1-2 SS valve fluid pressure (acting on the end of the 3-4 shift valve) to exhaust through the solenoid. Low fluid pressure acts on the solenoid valve fluid circuit. Spring force, acting on the opposite end of the valve, forces the 3-4 shift valve to move.