This is another way to compress a rubber ink sac.  There is a small knob on the end of the barrel, which turns a wedge-shaped metal bar inside the barrel.  As you rotate this bar, it twists the rubber ink sac round, compressing it.  To fill, immerse the nib in ink, twist the knob to the left, then return to to its closed position.  Wait a few seconds for the ink to be drawn in.  Lift the pen out of the ink, gently shake off the surplus drops and clean the nib with a tissue.