So if ballistics matter and the glande doesn’t move in a straight line, here’s how a slinger controls initial velocity:
-Rate of rotation
-Forward motion of the arm (and the slinger)
-radius of rotation (sling plus bent arm)
Of these, the second one is the least significant, maybe adding a few meters per second at most. That means #1 and #3 are the big ones.
Let’s look at #3 first: as boundaries, the sling can rotate around the wrist or the shoulder with the arm fully extended. Let’s say that’s a possible variation of 0-80cm which can be added to the length of the sling itself. With an 80cm sling you can double the speed by doubling the radius of rotation at a particular rotation rate. That gives a slinger a lot of control over the speed of the projectile even with the same throwing style and rotation rate. Realistically, this would be more of a fine tune control of the radius based on slight bends of the elbow.
Now for #1: I said previously that a reasonable rotation was 1-4 Hz. If you are swinging around the shoulder at 4Hz with a 1m sling, the max linear speed is about 45m/s. That’s on par with the velocity estimates that people are seeing for long distance throws, so I think we’re in the right ballpark.
So to sum up: how hard you swing is the most significant factor with a variability of up to 400%
Fine tuning happens at the elbow, but different styles may change the speed by up to 100%
And that little lunge forward is good for squeezing out a little extra power.
All numbers are rough order of magnitude at best, and this is obviously oversimplified