The stay and the trigger work together during the wind-up to accelerate the ball. At release, their jobs diverge. The stay is held firmly, while the trigger is released. At this moment the trigger is the catalyst transitioning the pouch from the holding job to the releasing job... These contradictory jobs are described in more detail above. It is the precise moment of the release that is the most difficult thing about slinging.
Moore attention will be given to specific techniques is subsequent posts, but for now it's important to note that the trigger must be released on the tangent. That is, exactly 90 degrees from the ball's intended direction of travel. This is not intuitive and difficult to master at first. But as with most things in life, it can improve.