Did you know the granny-shot is statistically more effective when shooting free-throws than an overhand shot. Learn more on this Moment of Science.
It works kind of like a soccer net. When you kick the ball into the net, it absorbs the energy of the ball and eventually stops its forward motion.
The windmilling motions of the arms are also fast forward rotations. Making some parts of the body rotate faster causes other parts of the body to rotate more slowly to compensate.