Re: Moving with your center
I agree with Ahmad. Read what he says carefully.
I could add that if you concentrate on relaxing shoulder only you may forget that you should also be relaxing your arm and hands too.
If your so intent on relaxing shoulder only that can make your arm tighten and you'll find you are actually drawing energy in rather than extending. T he feeling you should have is more a feeling or intention of offering or giving.
Very similar to Ahmed I could advise imagining you are holding a saucer of water which you must not spill. These type of creative things
definitely help you put your ki in your hands and help you relax the arm in the beginning.