The Sager is sometimes difficult to explain. There is a narrow tube in the middle of the barrel. That is the central fill tube for sucking up ink from the nib to the back of the pen. The plunger has a seal against the inner barrel as well as the central fill tube. Pull the plunger back. Then, with the section in the ink bottle, you push down. As the plunger goes down, the back end of the fill tube starts to suck up ink, which fills the space above the piston/plunger head.