Originally Posted by Attila_the_gorilla:
1) "All other factors being equal, a more closed racket face does increase topspin. It allows for asymmetrical force on the top half of the ball, depending on the nature of the incoming ball."
True, but only if we look at a very minor spin speed here, and only with a much greater than racket angle of 11 (Fed) to 15 (Nadal) degree angle as tennis players do. Try this: lock the racket at 11 deg and shoot a horizontal ball to it, and can you observe any spin on the impacted ball? Almost none.
So, in math equation, this will product a none zero spin nonetheless. However, no body can close the racket with 30 degree or more to generate any observable spin and still be able to lift the ball over the net -- this guarantees the ball will be at the bottom of the net.
2) "Now if you close the racket face slightly and block the same horizontal ball, it will result in the ball coming off the racket downwards and with topspin. Why? Because of the asymmetrical force aplied to it. The top part of the ball is blocked, while the bottom part keeps going, which results in the spin."
If you close the racket face slightly. How much? At 11 deg as Fed did in the article? At this small angle, there is probably 0.0001 RPM spin measured by very sensitive equipments.
To sum up: The main reason one needs to slightly close the racket face at impact is to lower the trajectory of the ball, not for spin generation. We all do this when caught at no-man-land haft volley, and this keeps the ball low over the net so the other guys couldn't smash the weak high ball, and not for generating spin purpose.