Apparently, there are different ways to do it.
"Underhand Method:
3) This method is basically a softball-like pitch. Swing the sling back and forth until you feel there is enough momentum to complete an entire loop. In the underhand method the rotation should come from your knees, up in front of you, then over your head, then behind you. When the sling has rotated back to your knees, lift up your thumb and the rock will fly out.
Overhand Method:
3) This method is basically a baseball-like pitch. It is a rotation of the shoulder and elbow. Swing the sling back and forth until you feel there is enough momentum to complete an entire loop. In the overhand method the rotation should come from your knees, up behind you, then over your head. When the pouch circles over your head, lift up your thumb and the rock will fly out."
"Underhand is good for distance, but you lose accuracy. People can throw rocks 400+ feet underhand with little effort. Overhand is generally better for close range, sub 150 feet. As for rock sizes, people use different things. Golf ball-sized rocks feel good as the additional weight allow you to feel the force on your hand and therefore have a better understanding of where it is going to fly. Often slingers make up for the lack of weight by doing grapeshot. Slinging grapeshot basically means piling in as many pebbles as the pouch can hold and launching that. This way, your target area is littered with a spray of rocks, making it highly effective for "light" targets."
From
Slinging.org
The model looks great btw!
