Well, Lynn... to the average beginner, what does that mean? OK, so (let's say I'm a rank noob) my center is 4 fingers width below my belly button and somewhere in the middle of body... HOW do I move from my center?
This is a very simple example of one way to teach a beginner to move from his center. Have nage stand in a natural stance. Touch the top of nage's head and say ‘concentrate here'. Put the fingertips of your hand just below nage's collar bone and push with increasing force towards the nage's spine and slightly downward. Note how much force you have to exert in order to move nage off balance. Return nage to natural stance.
Touch nage's one point and say ‘concentrate here'. Put the fingertips of your hand just below nage's collar bone and push with increasing force towards nage's spine and slightly downward. As you push, remind nage to ‘keep one point' and ‘extend ki'. With practice nage will be able to absorb greater amounts of force applied to the push than you initially used to push him off balance.
Once nage can perform this exercise successfully from standing it's quite easy to have him begin walking into the push and move you off balance. The exercise is performed as above but instead of standing and absorbing the push, nage literally walks into it. At first, while consciously avoiding concentrating on one point, nage will be taken off balance because his upper body will not move forward as he begins to walk. When nage switches to keeping one point he will begin to be able to walk through the push with relative ease.
The choice of metaphors you use to help nage visualize what is going on in this exercise is unimportant. You can talk about ground path, facial contraction, body connection, friction, angular momentum or whatever. Students will develop their own images to visualize the feeling of centeredness (what we call correct feeling or their strongest possible state). The feedback from the pushing is what allows nage to learn what feels dependable and then enhance and strengthen that feeling in order to absorb more force as uke gradually turns up the juice.
As with all paired ki exercises that involve testing it's important to remember that this isn't a contest. Uke's job is to aid nage by applying the correct amount of force in order to facilitate nage's ability to absorb the force and remain stable whether standing or in motion.