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

You can see, quite dramatically in this comparison, how the modern forehand is biomechanically different from the old classic forehands of yesteryear. The modern player on the bottom has his hand in a semi-western grip with his hand under the handle. This is ideal for lifting the ball. But the modern grip also lets you create more internal rotation of the shoulder which helps bring the entire hitting arm forward quite dramatically for increased leverage. Look how far forward the elbow is in the bottom image versus the top.

John McEnroe's arm is just slightly in front of his front foot. There is almost a straight line you could draw from his shoulder, down, his arm, to the racket on contact (but just enough forward to hit by his front foot). If you draw a line from the player's shoulder on the bottom to the racket and ball on contact, that line looks very different. His shoulder had rotated internally due to the grip, and the hitting arm has been pulled very far forward. So it looks more like a bent pipe with the elbow at a 45 degree angle and the wrist back and down at a 45 degree angle.

If you can imagine the ball pressing deep into the string bed and then rebounding back, you can get a sense of how important leverage is. There is very little mass behind McEnroe's racket because his arm is straight and it is not in front of his body. The player on the bottom, however, has the bent elbow and the more laid back wrist which provide mass behind the ball. And the contact point is in a much stronger position because it is so much further in front.

The real beauty of this modern position is what it lets you do next. And that is the next lesson.