Initially, the pivot point will be completely aft at full astern, it will gradually move forward as the boat moves. My vessel will just rotate around the stern initially, (assuming you have no forward motion) then the stern port movement will slow, then stop, then , assuming you have full starboard rudder in, it will eventually start answering the rudder. If you are moving forward, even though the pivot point is full astern initially, the forward motion will cause a yaw to port as the vessel continues to move forward.