Originally Posted by boramiNYC
the racquet head doesn't need to go below the ball to generate topspin. The racquet head must move upward as contact is made and afterward to generate topspin and this can be done without dropping the racquet head below the ball. And that's what I recommend to achieve the overall best form for E grip.
Actually, an off center hit with zero upward momentum would still cause top spin. However, bearing the ball's deformation, upward momentum does generate some spin -- it's just not nearly as important as what people usually think it is.
As for this advice, it is more or less what we should be doing with any grip. The swing path should rise, following a very shallow slope prior contact. But the comment is exaggerated: Federer does bring the racket lower than the ball's level, every time. It's obviously a nearly horizontal swing, but it's still rising from bellow the contact point.