Low and Reverse Clutch (B3) 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 M1 shift valve is shifted to the applied position, reverse clutch (23) fluid is blocked and low and reverse clutch (24) fluid exhausts through the valve allowing the low and reverse clutch assembly to release.