Jonathan, many thanks for this comprehensive answer. Your analogy with the child on a swing is excellent, and the concept of releasing the energy has given me much to think about with my dancing (fortunately, i'm good at physics!!). I find these 'mind pictures' invaluable.
I often watch myself on video (we video many of our practice sessions) and i keep commenting to my teacher that my dancing looks 'dull' and i suspect it's because i'm somehow blocking this free transfer of energy.
I am also concerned about my travel (i.e.i'm not using up enough floor in executing a given sequence of steps - we're not THAT bad though!!) and maybe this is a related problem.
My question was prompted by our experience in a comp last weekend (The Celtic Classic in Ireland) where, in the second round of a big event, we were almost 'eaten up' by the other dancers in a way which doesn't happen here in Ireland. They were pretty good international dancers and i felt like i was on court trying to play tennis with John McEnroe. In the 3rd round we were able to watch our friends whom we often practice with, and their lack of movement was very obvious.
I guess it's all down to practice, dedicated work and more practice. But i've learnt from my experiences in the tennis scene that practice focussed on specific weaknesses is much more effective than just hitting a ball around.
Many thanks for your reply which highlights once again what a great site this is.