Low & Reverse Servo
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.
When the 1-2 shift valve is held in the upshifted position, an exhaust port is opened. Fluid in the Low & Reverse band reduce circuit exhausts at the valve while spring force, acting on the piston, forces the servo downward. The downward movement of the servo enables the release of the Low & Reverse band.