Remember the Chilton and Chiltonian? Sheaffer made it even better. Now you pull back the end of the barrel. No need to cover a hole. It is all sealed. When you push down, the pressure inside increases and compresses the sac. But the real magic was the clever idea to add a small groove in the mechanism that breaks the air seal at the end of the stroke. So no finger needed. At the end of the down stroke, the pressure is released automatically.
Note the short groove near the end knob. This is the groove that breaks the seal at the end of the stroke.