[KSagal]

I believe you may be thinking of the 'y' shaped stick with a rubber band across the uprights, as in a modern slingshot.

The true or older more original definition would have been of a leather strap with a pocket for a projectile (usually a rock) in the center. You hold both ends and the pocket is at the center of the sling that is made from both ends being suspended.

Then the slingshot is spun overhead in a similar manner to a bolo or other stringed weapon, and one end of the sling is released, allowing the shot to fly.

Think of David and Goliath.

In relation to this application, I was thinking of a tool that employed centrifugal force (or the release of energy made by that force) to do some of the work being contained in Kamen's device.