The Sling. Simple in concept, but the design of an effective sling is subtle. The sling has two contradictory jobs.
Sling's Two Jobs:
1. Hold the ball
2. Let the ball go
The very best sling designs excel at the transition between the first job and the second. So while the cradle of the sling must be secure enough to keep the ball from rocketing off in some unintended direction during wind-up, that same cradle must also open quickly when the trigger is released. If the ball has to spend it's momentum pushing or rolling its way out of the cradle it will lose exit velocity and accuracy. In this way a good sling is known in the same way that relay races are won... that is... in the handing off of the baton.