Sorry this might be a stupid question, but I have found that some setups give the ball more dip and some setups give a different kind of spin. I think this is associated with dwell time. I notice when watching some college players and especially roger federer the ball just dips before the line. They aren't even neccesarily hitting with a western or even semi-western grip but the ball dips tremendously. Is the all technique or does string selection take a part in this? Tension?