The plunger filler is simple to use and is also a very elegant filling system for fountain pens. To use it, pull back the plunger. Put the nib in ink. Press down briskly, and at the end of the down stroke the pen will fill.
Inside the pen, the plunger rod is sealed to the back end, and the plunger head has a round gasket that seals to the inside of the barrel. As you push down, it creates a vacuum above the gasket. Near the section end, the barrel internally gets wider, breaking the seal to the gasket. So at the end of the stroke, ink is sucked in by the vacuum left behind from pushing the plunger down.
In the US, it is most typically thought of as a Sheaffer filling system. However, the earliest usage (or earliest common usage) is on hard rubber Onoto pens. Onoto made pens with this filling system for many years, starting around 1905, and carrying it into celluloid pens as well. The plunger filling system is also seen on some Eversharp Doric pens and others.
Here is a video of the pen in use courtesy of https://paperwantsapen.com/ . Talk to him for all of your plunger filling fountain pen repairs and restoration, especially Sheaffer plunger filling fountain pen repairs and restoration!
In the video below, the first few partial presses of the plunger illustrate how a well-restored Sheaffer plunger, if only partially pressed, will push back out by itself due to the strong vacuum created between the back end of the pen and the plunger gasket. When the plunger is depressed fully, it reaches an enlarged part of the inner barrel. At that point, the vacuum draws up ink into the barrel.