Hump Phenomenon
WARNING: This page does not describe the selected car, but rather 12 other vehicles, including the 2014 Subaru XV Crosstrek, 2014 Subaru Impreza WRX, 2014 Subaru Impreza, 2013 Subaru XV Crosstrek, and 2013 Subaru Impreza. However, it is still accessible from the selected car via links, so may be relevant.
Silicone oil is heated and expanded as differential action continues. The oil in the viscous coupling expands and compresses the inside air to cause direct plate-to-plate contact or a non-viscous operation to occur; this phenomenon is called hump.
The hump eliminates the rotating speed difference between the center differential case and the rear side gear (or locks the differential), so soon after it has occurred, the internal pressure and temperature drop. The viscous coupling then returns to the normal shear torque transmitting operation. (The hump phenomenon does not occur under normal operating conditions.)