Lampi when sailing a board backwards you are not using the fin.
Practiciing with out a fin gives you a better feeling for keeping rig and body in balance and moving smoothly. It means you can practice in very light winds sliding around. When you are powered up and using a fin,in fact when you are sliding you are not using the fin.
You can start with any of the basic arial moves, in fact the volcan is probibly the hardest as it requiers a rig change.