Part of a
fountain pen that
fills using a
sack mechanism. The pressure bar is a
flat piece of
metal (usually
brass) that sits between the
sack and the
barrel of the pen. It is
manipulated from an
input on the
outside of the
pen (typically a
button at the end of the
barrel, or a
lever on the
side of the
pen).
This manipulation either moves or bows the pressure bar, which, in turn, compresses the sack. Everything in the sack is expelled, including ink and air (be careful where you point that thing!). Once the pressure on the bar is removed, the sack is allowed to expand. A vacumm is created, drawing in whatever the surrounds the nib (again, ink, air, or water).
As mentioned buttons and levers are the most common methods of manipulating a pressure bar.