WARNING: This page is about a different car, the 2008 Subaru Tribeca, 2008 Subaru Outback, and 2008 Subaru Legacy. However, it is still accessible from the selected car via links, so may be relevant.
The line pressure which engages shift clutches to create 1st to 4th speeds, is regulated to a pressure required at each clutch according to the input torque.
Fig 1: Identifying Graph (Line Pressure Control In Normal Conditions)