A metal tip houses a tiny, rotating ball that seals the ink reservoir against the world. When you press the tip to paper, surface tension relaxes just enough for the ball to turn, dragging viscous oil-based ink onto the sheet while keeping the remainder tucked safely inside the tube.
The ink path is one-way: the ball spins outward, but gravity and an internal spring-loaded sleeve keep it from backflow. This balance maintains a clean line until the reservoir runs dry.
Out of use, the tiny ball stops rolling and the ink puddles at the tip, forming a seal. Cap or retract it to avoid drying. Choose a thicker refill when you need longer writing life, and discard if ink dries into a clog—gently twisting the tip can often dislodge a stuck ball.