Differences between a Kyubell and Dumbbell
Both a dumbbell and Kyūbell are simple versions of a handheld weight. Dumbbells offer a scalability of load in many different exercises. Kyūbells offer scalability of load like a dumbbell but they also allow users to target different force curves within common movements. Manipulation of how force is applied to a muscle is one of the many applications of the Kyūbells.
Understanding the different positions
Manipulation of force is the #1 goal of any resistance training implement. Force is a vector quantity that has both a magnitude and direction. Force is a non-linear expression that results during push and pull movements. The number one way to influence force output in resistance training is through progressive loading. The second way to influence force output is the position of an implement relative to the lever arm (we often refer to this as changing the mechanical loading of an exercise). Kyūbells can both influence force through progressive loading but also through changing position in common exercises.
The center mass position is the most stable of the various positions. Due to the stability of the center mass it is the position that can be loaded the heaviest (in terms of absolute load of the implement).
The rack position allows uses to target larger force curves (muscle will be under load longer) in various upper body fly exercises. Some users also feel the rack position is easier on their joints when doing pressing exercises though this is completely up to your individual biomechanics and injury history.
The long leverposition is the most demanding from a mechanical loading perspective. While the inherent absolute load of the Kyūbell will be lighter relative to other positions, this position produces the longest lever arm in most exercises.