This is a very interesting and perhaps over-engineered syringe filler. The goal is to be able to push the plunger rod back inside when done, but still keep ink in the barrel. This is accomplished by the piston seal being able to slide partially up and down the piston rod. The piston rod is perforated. During the pull stroke, the piston is sealed to the rod, and it acts like a standard syringe filler. On the downstroke, the rod slides down so the perforation straddles the piston head. This allows for a downstroke that transfers ink to the space above the piston head seal. Repeat a couple of times and the pen is full.