Examples include 1940s Sheaffer models and many Onoto pens.  The piston mechanism varies, but all fill on the DOWN stroke by causing a slight vacuum behind the piston, which is released at the bottom of the stroke, causing ink to be sucked up.  To fill, immerse the nib in ink, pull the piston rod out to its fullest extent, then push it down again and wait a few seconds for the ink to be drawn in.  Note that most fillers lock down by screwing in a couple of turns.  As usual, shake off surplus ink and clean the nib with a tissue.