Low & Reverse Brake: Function
WARNING: This page is about a different car, the 2013 Subaru Forester, 2012 Subaru Forester, 2011 Subaru Forester, and 2010 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.
When the 1st gear of manual mode or reverse is selected, the pressure from the control valve is applied to the low & reverse brake piston. The piston then presses the drive and driven plates together and causes the low clutch drum to lock.