I've interpreted your question in two related ways.
The first has to do with the open/close aspect of Tai Chi. If you leave your breathing alone and move with it, you may be able to feel the ebb and flow of the energy. Allow the movements to be smooth and even, and move as though you are in deep water moving back and forth with the tide.
The second has to do with the basis of timing. This is much easier to show than to describe, however, I'll try to express it briefly.
The principle behind the timing of all the moves in Tai Chi is a simple one, and it is based on yin/yang ideas regarding mixing and matching the means and extremes. The mean is the middle, the extreme is the furthest point.
For the three dimensions then: the up/down plane, the left/right plane, and forward/backward plane; in each dimension there are the extremes and the mean.
If a hand is at your waist, say, in 'Repulse the Monkey' that is the mean for the forward/backward plane. Your right hand is at the extreme, for the up/down plane, when it is over your head in the final posture of 'White Crane Spreads its Wings.' One hand may be high and the other low, like in 'Strike the Tiger," left and right. Both hands may be up and out, like in 'Double Punch to Temples,' (also called 'Double Wind.') One hand may be at the forward extreme while the other is at the back extreme, as in part of 'Repulse the Monkey.'
The position, then, of a hand, or a foot, in any posture, may be described in terms of the mean or the extremes on all three planes. This also applies to describing the positions of the elbows and knees.
Now, applying this to movement as well as position: As each part of your body moves from the mean to the extreme, or from the extreme to the mean, the others do the same. Sometimes both hands move out, or one hand goes up while the other goes down, or both hands move to the left, or one hand goes out while the other comes in. Nearly every possible permutation is employed. There is a constant ebb and flow.
Two examples for incorporating the feet:
The transition from 'Grasping the Sparrow's Tail to the Left,' to 'Ward Off': as you shift all your weight onto your left leg, your right hand is still moving to the right, and the left hand is moving to the left. As soon as all the weight is on the left leg, you bring in your left hand, your right hand, and your right foot.
In the beginning of the transition from 'Play the Harp' to 'White Crane,' at first, both hands move downward toward you, then when the left hand comes up, the right foot is lifted up.
Basically, things move together or apart, reaching the mean or the extreme at the same time.
At first, don't try to incorporate these ideas, but observe. Do the long form and watch how much of it you already do. Correspondence is across the body, too. It's easy to see the left hand and the right hand coordinate, or the left elbow and left knee, but look for the left hand and the right foot coordination (and vice versa), look for right elbow and the left knee coordination, etcetera.
Watch someone who does it right.
It's built into the structure. Let it flow.
[This message has been edited by DavidJ (edited 03-14-2001).]