I know of this work and often replicated the technique.  It was not until after using this natural augmentation to the 'way of the sling' for some time, that I came to know of Michelangelo's depiction.   Of course,  there is no way to be certain if David himself used this convenient way of keeping the ammo in the pocket until bringing it into action......Although I like to think that either Michelangelo's personal experience with the sling, observations of contemporary slingers, or perhaps deep contemplation brought him to reveal such a realistic conclusion.

In my teens,  I would situate the sling pocket as shown and hold it in my left hand and toss it over my shoulder and let it swing down behind and then I brought my right hand into play for a diagonal underhand launch.  With the advantages gained in mastering the overhand launch....I now lean toward such a style as, perhaps, more likely used by those adept slingers in David's day as a close range technique.

The other statue I was talking about was by Bernini.  Now that one look like david was just about to throw the rock!

I found pictures of both statues and many other good stuff at Web Gallery of Art. www.wga.hu/

I resemble that remark.

More than likely Michelangelo was depicting art, and was more concerned with religious symbology than historical techniques...