The single most important item is that the hole diameter in the propeller be large in comparison to the nail, about twice as large, say. It basically then can be pictured similar to a twirling whoop on a stick, the side force of the finger or thumb on the notched stick forces it to the side as each notch is hit thus giving it a repeating side jump to pivot the propeller in one direction or the other. A nicely balanced propeller will spin just fine and it actually is not critical to have it heavy to one side, because the hole diameter always allows one side (slightly longer) to drop during the jerking motion which occurs. To observe the effect, the propeller attached to the end of the ribbed stick must not be pinned at its exact centre of mass, i.e. off-center or loose. For a functioning whammy diddle, there should be a "preferred" angle (i.e. when stationary, if the propeller is rotated around its axis and then released, it should return to its lowest-energy rest angle relative to the ribbed stick.) Rubbing the ribbed stick with another stick along the grooves creates a vertical alternating driving force F y {displaystyle F_{y}} F_{y} acting on the centre of mass of the propeller: