#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 positioned against the Low & Reverse band apply circuit. The checkball allows Reverse band fluid from the Low & Reverse servo piston to enter the Low & Reverse band reduce circuit. Fluid in the Low & Reverse band reduce circuit is then routed to the 1-2 shift valve where the fluid exhausts.