Originally Posted by akamc
Hitting a falling ball gives you a higher vertical velocity component of contact relative to a rising racquet, generating more topspin in your serve. It's all about finding your best personal compromise for what you are trying to accomplish.
very very sketch.
On the other hand, I have a feeling you would have to reverse the downwards momentum of the incoming ball when brushing upwards on it, making it harder to hit topspin.