Originally Posted by tank_job
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.
Actually not "sketch" at all. It is well known that a rapidly dropping toss imparts topspin to a "flat" serve and thus increases it's consistancy, pehaps by about 10%.
H Brody is the acknowledged expert in this area.