We have seen that gravity is the force that lets you initiate your stroke effortlessly. By letting the double bend hitting structure "free fall", with the aid of gravity, you start the stroke off effortlessly and then use this "free" momentum to help propel the next stage of the stroke, which comes from ANGULAR MOMENTUM.

In the animation on the right, watch how the "free fall" turns into rotational energy as Acasuso unwinds, or turns his entire upper body, into the ball. I drew a line straight down the middle of Acasuso's body to show the axis of his rotation. He rotates his upper body, along with the double bend hitting structure, around this axis into contact.

This rotation around an axis is the exact same force figure skaters rely on when they twirl on the ice. And if you look carefully, you can see how Acasuso tucks in his left arm as he rotates around - the exact same way a figure skater tucks in his/her arms to increase the speed of their rotation (special thanks to one of my students, Tom Antion, for pointing this out to me.)

This combination of "free fall" and angular momentum leads to tremendous momentum into contact with very little effort. Rather than using your own force to "swing" the racket into contact, the racket is quite passive as it gets lowered and then pulled around into the ball by the upper body and double bend hitting structure. This is why pro strokes look so effortless, yet are so powerful - they tap into the force of gravity and the power of angular momentum to generate speed rather than using their own muscular force.

To take it one step further, angular momentum is getting magnified here. As the upper body begins to rotate into the ball, the butt cap of the racket leads the way. So this leaves a SECOND force of angular momentum - the inside out path of the racket - to add even more momentum into contact. In the Acasuso clip you can see how the racket moves inside out just before contact.