** Compatible with ToonBoom Harmony 12 **
This rig uses expressions to simulate a deformer of a deformer. By deforming the central curve you control the shape of the other two curves, which can be further adjusted by the controllers (in red). Move the red circle to move the deformer vertex (dragging it with the mouse will give unexpected results depending on the rotation of the arm, so its best to use the keyboard) , rotate and scale the arc shape to adjust the green handles.
It is easy to duplicate this structure and change the drawing so you can use it in your rig.