The Snorkel filler is a further development on the touchdown but is different in that it is a "clean filling" pen.  To fill, unscrew the knob on the end of the barrel, keeping slight downward pressure - you will notice the snorkel tube being extended from under the nib.  When the snorkel tube is fully extended, you will feel a "clicking" as you turn the knob, indicating that you can now pull out the filler tube as on the touchdown - it comes out about 5 cm.  Dip the snorkel tube in ink and push the filler tube back down again fairly quickly.  Air pressure compresses the sac and then leaks away to allow it to re-expand.  Wait a few seconds for the ink to be drawn in.  Screw the knob back with slight downward pressure and the snorkel tube will be retracted, then lift the pen out of the ink and clean the nib off with a tissue.  Done.