This sounds like something they do at Tom House's facility in San Diego. The pitcher actually leans against a device that has pads for the front of the hips to rest against and pegs for the back of the ankles to lock under. The device puts the pitcher in a position where they are leaning forward. To throw, the pitcher arches the low back to raise the head and shoulders upright. The pitcher then reverse rotates the shoulders before throwing the med ball into a small trampoline. At release, the torso bends forward but must be raised back up to catch the ball coming back off the trampoline. The ball used is, I believe, a 4lb ball. It is NOT thrown at full force.
I believe it helped to exagerate the loading of the shoulders as well as the forward rotation of the shoulders. It is basically a core strength and flexibility exercise. It doesn't really have anything to do with the arm.
I don't know about going to your natural arm slot. But what I do know is that you'll adjust to a position of strength where you can safely handle the extra weight. This normally means bringing the hand closer to the body.