Good players today crush the ball not through "racket head speed" but by transferring the power of their upper body, arm, and shoulder into, and through, the ball. They do this by working the ball with the racket, hand, and arm together - as a unit - through contact. Think of it like pushing open a heavy door, or pushing over a heavy barrel of water. First you would align your body behind your hand and arm, and then you would forcefully push and accelerate with the hand, arm, and shoulder together. This is the way you shift your weight into a heavy object to move it. That's what top players do as well on their forehands.

Most players and coaches work, consciously or unconsciously, on a "racket head speed" model. When we talk about "racket head speed" and "peaking racket head speed at impact", we are completely missing the dynamics of high level tennis. The racket, as Doug King argues, is a connector between the ball and our powerful arm/body. It's like the clutch in a stick shift car - the clutch connects the powerful engine to the wheels, allowing the engine to power the car. In tennis the racket connects your body to the ball and allows you to use a powerful weight shift to push and lift the ball. It is nothing like the "swing fast/racket head speed" model most of intuitively work with.

If you can think of the racket as a connector between your body and the ball, and you can think in terms of "align and push/lift" then you will be well on your way to high level tennis. If you think in terms of racket head speed and "peaking racket head speed" at impact, you will completely misunderstand the role of body, racket, ball interaction. Telling someone to "peak their racket head speed" on impact is, in my opinion, the best way to ruin a person's chance of ever hitting the ball at a high level. It places all your empahsis on the racket instead of the body. It places all your emphasis on speed instead of on the power and force of the body. And it completely misses the importance of properly aligning ball, racket, and arm/body.

Top players work on alignment first - what Oscar Wegner calls "finding the ball" - through torso rotation and a lifting of the arm. THEN, when everything is lined up, they acclerate with the mass of the arm by lifting upward and pushing forward - creating tremendous spring and pop off the racket.