The very best writer on this topic is Rolf Clark in his wonderful book "Breakthrough Tennis". Chapter 10 is entitled "Timing and Skinny Twelve-Year Olds" and contains the best description of timing I have come across. He starts out by saying that timing is really about letting things move in their most natural way. He uses the example of a child on a playground swing: "You wouldn't push when the swing is still coming towards you, nor stop it at the top of its arc and then let it go. Those disrupt the rhythm. Instead, you help the swing move its own natural way" (75).
I mentioned on the previous page that the stroke actually begins as you start to lift the racket and arm upward and backward in a gradual arc. This gradual motion upward and backwards begins the continuous flow that peaks at the top of the takeback before falling with the aid of gravity. Rolf uses Newton's Law of Motion to explain why top players do this, rather than "take the racket back right away" as all the instructors tell us to do.
"Recall Newton's Law of Motion: For every action there is an equal and opposite reaction. To roll a bowling ball down the alley, you first swing it back behind you, then swing it forward and then let it go. There is no delay between the action and reaction. The action causes the reaction. A delay would interfere with the smoothness of the sequence. In tennis you swing the racket back so you can swing it forward, neither rushing nor delaying. That's action and reaction at work. If you bring your racket back too early, you end up delaying the forward swing while you wait for the ball. If you react late to the ball you compensate by pulling the racket back suddenly and then forcing it forward. The result is poor timing and a poorly hit ball. You wouldn't and couldn't do that with the bowling ball. The ball is too heavy. You swing it back and let it swing forward when it's ready. If you don't -- if you delay it at the back of your swing, or if you jerk it back too fast -- you'll hurt your shoulder.
If the racket is to meet the ball at the right time, then the forward swing must react naturally to the backswing as Netwon says. It should follow the backswing without delay. That, in turn, means the backswing must start at the right time -- not too late, nor too early. Practice the action/reaction sequence at home. Hold the racket in front of you and imagine a ball coming at you. Bring the racket back and then swing it forward to meet the mental ball with natural swing timing." (82)
-Rolf Clark
Breakthrough Tennis