The rudder cam (actually the RCU) is mounted between Rudder and the linkage to the pedals. It enables rudder stabilization when no action is applied to the pedals.
Prior to take off, the pedals must be centered and the rudder cam released by action on the rudder trim switch to allow the rudder to "stabilize" in the neutral position.
When trimming the rudder, you are in effect changing the position of the rudder so the cam is released to allow the rudder to go to the new position then it re-engages to stabilize the rudder in the new position.
A good reference is the flight control section of the FCOM.
Hope this helps.