One of my favorite forehands belongs to former WTA pro Nadia Petrova. She was one of the first pro players I ever filmed and she just blasted the ball with a powerful windshield wiper motion. It wasn't until later, when I filmed her from behind, that I could see how the key to her windshield wiper forehand was the way she PULLED the racket through the slot very early in the stroke.

By rotating her hand backwards as she rotates her torso forwards, she is creating incredible LAG of the shoulder, arm, and racket. In the still on the left, imagine someone is grabbing her racket and pulling it backwards and to the left while she opens her torso towards the ball. This is a great exercise for you to try if you need to develop lag in your forehand. As you start to uncoil your shoulders towards the ball keep your racket back near your right hip and twist the hand backwards. Then have your coach or someone else tug backwards on the tip of the racket. Then pull the racket, butt cap leading the way, through the slot. You should feel the racket being pulled through the slot, creating tremendous lag that can be released into the ball just before contact.

Once Petrova has pulled through the slot, and her elbow is in front of the plane of her body, she is ready to release the lag and have the tip of the racket travel inside out into the ball, generating a burst of racket head speed. As she releases the wrist, she continues to pull her arm forward to the ball. In fact, the release will also propel the arm forward into the ball as the racket travels an inside out path. The racket will "catch up" to the rest of the arm on contact, with the wrist going from a 90 degree layback to the forearm to about a 30 degree angle.

In the animation on the right you can see how the tip of the racket whips around into the ball. But the arm is still pulling forward as the racket whips around. The racket, which has lagged behind and has been pulled, is now going to "catch up" to the arm in an inside out motion that generates a burst of speed. Now that everything is back in sync, the arm can take over and lift and drive the ball with the muscular windshield wiper motion.