I've had one experience with a sling like is in that picture. I was about 7years old on a family camping trip. I was a little bit downhill from our campsite trying to toss little rocks over the small creek as my father watched while making breakfast. The "aim" was to hit a boulder on the other side. Tried one: swing, swing, swing, THROW..... hmm.. nothing. Rock fell out of the pouch. oops. Tried a few more times. Next one seemed to be in orbit, then made a little sploosh in the creek. I was encouraged. Final try, I didn't see or hear anything until I heard an expletive from my father. Ran back up the hill to find that my little rock had somehow made it behind me, up the hill, and landed smack dab in my dad's oatmeal. Thus endeth the "practice".