The #6 Checkball (Low/Reverse Band Apply/Reduce)
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
The #6 checkball is located in the valve body. The checkball is seated against the Low & Reverse band reduce passage. The checkball routes fluid into the Low & Reverse band apply circuit. Low & Reverse band apply is then routed to the Low & Reverse servo piston and actuates the piston.