I posted a video of a simple medicine ball progression, in which I tried to demonstrate the dissociation and subsequent linkage between the hips and torso during a rotational movement. I think from the angle of the camera, you get a pretty solid idea of how the hip-hinge pattern plays such a critical role in rotational power development. If you pause the video to the starting position, kneeling or standing, you see the same hip-flexion angle of a base athletic position. This position is also reminiscient of a golfer at address, a baseball batting stance, or even any athlete prepared to accelerate laterally (although when I'm standing, my feet are much narrower - more similiar to a golf swing in this case). The same coordination between the lower and upper body must take place to accelerate the ball (or body) towards the wall.
An athlete must develop a bilateral hip-hinge-extension/RDL pattern in order to generate rotational force effectively. Glute bridging, RDLs, KB swings, pull-throughs, and other various hinging-type exercises should be used to develop the pattern. A more extensive medicine ball progression would include: underhand tosses (similar to KB swings but you release the ball towards a wall), hips-square to the wall rotational passes (kneeling to standing) and then hips perpendicular variations (like the video).