Actually the rudder cam box is mounted between the rudder trim and the aircraft frame (aft bulkhead, not pressure bulkhead). Due to aerodynamic design, some vibration/flutter occurs at the rudder level, both rudder plane and spring tab. The rudder cam is a box with a set of springs to reduce or cancel out this flutter.
Because you need to trim the aircraft rudder for different positions (power settings change or assymetric flight during engine failure), the cam needs to be disengaged momentarily until a new trim position is achieved.
Once the trim switches are released, the cam re-engages, again protecting from some possible flutter.
The reason you want to center the cam before takeoff, is to have an equal amount of spring force on both sides, so in case of an engine failure, the initial rudder pedal pressure would be consistent, whichever left or right engine fails.
Before takeoff, just flick top rudder switch to the left to center the cam, without actually moving the trim motor. (You can practice this at the gate on the ground, and with light pressure on a rudder pedal, feel the cam releasing and re-engaging.)