When you do flat serves, do you hit it totally flat or do you add a little spin? because everytime I try to hit hard flat serves, most of the time the ball would either go out or net, but when I add some spin, the ball is more likely to go in, but the serves aren't as fast. I wonder how do you guys hit a really hard flat serves and still in? do you add a little spin or 100% flat? anyone have a video of a pro hitting flat serves?