Once proper contact is established by "finding the ball", and you get a feel for "lifting" the ball, the next step is to turn your forehand into an explosive motion of the arm and hand, driven by the powerful shoulder muscle. Watch how Hewitt's right shoulder powers his windshield wiper forehand by pushing, lifting, and rotating all in one powerful, explosive motion. Imagine there is a "heavy barrel of water" as Oscar describes it, and Hewitt needs to powerfully lift it and push it over.
What is fascinating here is how the racket holds back so that the arm and shoulder can do the explosive work of pushing, lifting, and turning. It's like in boxing, the glove is what connects in a punch, but it's the shoulder that is driving it. Same thing here. The racket only provides power through the shoulder that is driving it, which is why the strings of the racket always face the net. This move can be described as more of a "shove" than a "stroke". But it's a shove that uses the full range of the shoulder - pushing, lifting, and rotating.
The modern semi-western and western grips make sense when you see that the stroke is not a "hitting through the ball", but instead a powerful pushing, lifting, and turning motion. Notice how Hewitt's hand is under the handle and his knuckles face the net. This hand position is perfect for pushing, lifting, and turning from the shoulder. This animation really captures what Doug King observed in his incredible article "The Leverage Game: Get Behind It"
People often have difficulty changing into the more modern grips because when they do this they are actually using totally different swing models. A semi-Western grip is based upon leverage and spring from the body while a Continental grip is based upon swing of the racquet and a steady body. The roles of the body and racquet are reversed in these systems.- Doug King
The Leverage Game: Get Behind It
Hewitt's finish position is so telling. He has used his shoulder to push, lift, and rotate the arm and hand through the ball. The racket and arm are always connected to the pushing and lifting of the shoulder. In fact you can see an imaginary "box" in front of Hewitt's body, formed by his forearm, racket and arm. His upper arm is parallel to the ground from the shoulder's lifting motion. His hand and racket are in front of his body from the pushing motion. And the tip of his racket faces the left side fence due to the rotating motion from the shoulder, which has rotated a full 180 degrees.
Watch the animation several times to get a sense of how the hand and racket hold back while the shoulder drives, lifts, and rotates the arm. Also imagine your shoulder doing the work of your forehand, not your arm and racket. Model the same positions you see in the Hewitt animation. This will take your forehand to a new level.