Reverse, 1st Shift Solenoid (SS) Valve Assembly (S1)
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.
Line (2) fluid passes through orifice #3 into the S1 signal (14) fluid circuit in preparation for a shift. The normally open reverse, 1st SS valve assembly is OFF, allowing any fluid pressure in the S1 signal (14) fluid circuit to exhaust through the solenoid.