While I was digging around trying to find a picture source for Staffslinger I found a picture from a German manuscript from 1616, "Romanische Kriegkunst" by Johann Jacobi von Wallhausen which featured an interesting technique



In the upper left if you look at their hands you can see a stick on the retention line and being held in both hands and judging by the size of the projectile compared to the smaller ones by the slingers to the right.

Posted it in general because it's less about the history it just happens to have a historical source, otherwise I would have put it in project Goliath

The stick slinger seems to do one sweep around the body with the heavy rock.
It looks like the soldier on the right uses a very primitive strap sling to sling with, and that combined with a tight grip. I experimented with that combo and cannot vouch for it.
His style is some sort of overhand, could be figure 8 in the best case.

@Kordward - Let me know if you want this moved to Project Goliath.  I agree that it could be justified in either board, so doesn't make any difference to me.  As the OP I'll leave the call up to you.

Definitely a cool find!