Shelf buttons are shortcuts to commands and set up quick actions to speed up common workflows.
Select a Shape or Shapes and then click the Duplicator icon. This will create a Duplicator and automatically add your Shape(s) as Input Shapes.
Add an Align Behaviour to the Composition and connect it to the Deformers attribute of any selected Shapes.
Add a Rig Control to the Composition.
Add an Animation Control to the Composition.
Clicking this Shelf item will create a default Rubber Hose Limb (the same command as Animation > Create Rubber Hose Limb). If you have a Shape selected the Rubber Hose will be connected to the Deformers attribute.