Boat static in slip. Put in forward gear. move rudder port and
starboard, stern moves accordingly. Put in reverse gear, again same
result. 'splain dis to me,Luci?

If you have a large enough rudder and you can swing it fast enough, you will
see some movement of the stern when static in the slip. In fact, the movement
will be exactly the same whether forward or reverse is
selected....................as long as the engine isn't running. :-)