The original discussion in this thread was talking about Michaelangelo's David, posed with the sling draped over the shoulder. This doesn't look much like an action shot, but Lobohunters setup for and underhandthrow works pretty cleanly starting from this somewhat "relaxed" looking pose.
The statue posted by ChuckRocks shows a much more dynamic posture.
I had trouble with the link above, or at least it's very slow to load, but we have some images of Bernini's David in the media section:
Google images also turns up lots of different angles.
I thought we'd talked about this one before, but I can't find anything in the search. It looks to me like he is setting up for an overhand throw, something that is also suggested by Bernini's other slingers.
David is holding the projectile palm down in his left hand, while pivoted to place his right foot forward. His right hand is drawn back low and behind his hip. The tension in the statue suggests the moment just before he releases the stone.
It seems like the most likely throw is a simple drop down on the right side while stepping forward with the left foot and whipping the sling over the top in a no-windup overhand. There is always room for an extra fig-8 type turn in there but the starting pose looks a little too powerful to me.
Let's hear some experiences with this windup...
Matthias