This is one of those things that everyone talks about but I'm trying to understand why/how it helps, because I feel that it does.
The ball makes contact with the strings for only a split second, so what does "driving through the contact zone" do if the ball is only interacting with the racquet for a split second? Logically whatever you do after making contact doesn't matter so I'm trying to understand why this piece of advice works.
If the "driving through the contact zone" means the often quoted "driving through the ball" by many coaches today, this could be a confusion topic. Coaches would give totally different answers, depend on how they understand the differences between traditional and modern tennis. Old school catches would ask you to drive through the ball to get the pace and control the direction. Things changed dramatically in modern times with the big racket head, so players can hit topspin shots with open grips and open stances, transforming the predominant linear hitting patterns to more violent, angular way of hitting tennis.
The basic thing is that we don't need to swing the racket toward the target line to get the good pace and direction control. How can that be?
If we compare javelin throw with discus throw, things will be more visual and easier to understand. In javelin throw, the hand and arm are throwing in the direction of the target, just like the old way of hitting tennis ball with closed stance and continental grip (and in current one-handed slice return and volley too). In discus throw, this way of linear throwing is absent, instead, we see the angular (rotational) swing with the hand far away from the body center, just like the modern one-handed forehand (wind shield forward swing motion). Through tennis is not a throw sport (we hit the ball using the racket, not throwing the racket to the target), the concepts/practices are the same.
Another sport that use more linear way to hit the object is a hokey. A hokey player usually hit the puck with drive through the ball motion. The difference here is the hitting object is very different. In tennis, the power of a shot depend not only on the pace of the ball, but also on the spin rate (bounce factor) of the ball; but in hokey, the puck is flat and not bouncing like a tennis ball, and a player cannot do tennis like rotational body movement on ice.
By the way, sometimes players do use driving through the ball motion to change the direction to hit down the line shots. Nadal's famous helicopter forehand shots have some driving through the ball motion, but he also add a vertical swing up motion to increase the spin rate.
