1-2 Upshift Checkball (#8)
WARNING: This page is about a different car, the 2008 Isuzu Ascender, 2008 GMC Envoy, and 2008 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Exhausting the 2nd clutch fluid pressure unseats the ball and is routed through the 2nd fluid circuit.