3rd Gear Band (B4) Servo Piston Assembly
WARNING: This page is about a different car, the 2009 Pontiac Torrent and 2009 Chevrolet Equinox. However, it is still accessible from the selected car via links, so may be relevant.
When the U2 shift valve shifts to the applied position, 3rd gear band apply (46) fluid and servo apply (45) fluid exhaust through the valve allowing the 3rd gear band servo piston assembly and the 3rd gear band assembly to release.