#11 Checkball (Third Clutch, Orificed Third Clutch)
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 #11 checkball is located in the case cover. This checkball is unseated by orificed third clutch fluid. This allows fluid to enter the third clutch fluid circuit.