Yes, that body posiotion helps a lot. Now try this. On some serves try to hit it so the ball hits your blade at 10 o'clock, or at 2 o'clock (where 12 o'clock is 180 degreee away from where the handle leaves the blade). You will get a lot of underpin at 10, much less at 2, and if you hit it at 4 o'clock you will start to get topspin, all from exactly the same motion.