Is this counterpunching, perhaps? I could be completely wrong, but when you're on the run, the ball (unless it's a drop shot) is usually hit with pace. By counterpunching you use your opponents pace, and when you do that you don't have to focus on swinging hard, you can focus on directing the ball instead.
Could be completely wrong, just a guess
"That's not Youzhny. That's Agassi with a Russian pj." --Armand