WARNING: This page is about a different car, the 2004 Pontiac Sunfire, 2004 Chevrolet Classic, and 2004 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
A spring force keeps the 2-3 shift valve in the downshifted position. In this position the valve blocks 2-3 drive fluid, in preparation for the upshift into Third gear.
Fig 1: Overdrive Range, Second Gear - With Electronic Range Select Mode