#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 held against the Low & Reverse band apply circuit. The checkball routes fluid to the reverse clutch circuit. Reverse clutch fluid is then routed to the manual valve where the fluid exhausts.