After reading Dave Dunipace's bent arm throwing article about ten times, I came to the conclusion that it might be possible that the technique is very reminiscent of the action of the trebuchet.
This is a side view of a counterbalanced trebuchet.
This image may better represent the motion of the arm and shoulder. Notice how the pivot point must move.
For our use picture a left handed thrower viewed from the top. Of course, the main difference is the human arm does not break over at the elbow, so you would have to picture this device with a stop placed at the sling arm that only allows the arm to lock out straight.
This stop action of the human elbow might help explain why the arm acceleration through and beyond the hit and release becomes so important in the bent arm technique. For acceleration to occur at the launch point (where the stone is held in the pic) the entire arm would have to come up to the speed of the sling arm. If not, there is no acceleration, or in other words the upper arm can serve only the slow the lower part of the arm and wrist.
Also, I think this may also show that the arm must reach full extension at some point very near the rip point.
Lastly, notice the arc. This device definitely does not travel in a straight line.