It is the great common denominator, the one tool that nearly every licensed adult in the United States will work with over and over again in their lives. The humble fuel nozzle is both ubiquitous and woefully underappreciated. Its basic concept and design are more than 100 years old, but changing fuels, vehicles, technology, and regulations have forced constant, unseen evolution. And still, a modern nozzle is beautifully mechanical, free of burdensome circuitry. When you squeeze the handle, the main valve opens, allowing fuel to flow through the body, down the spout, and into your tank. It shuts off automatically by a bit of cleverness. As the gasoline rushes past a venturi, it creates a low-pressure area that pulls air through a small port at the tip of the spout. Once fuel reaches the port, the air stops, creating a vacuum that pulls on a small diaphragm that in turn closes the main valve—simple physics that keep us all riding.

Source: MotorCyclistOnline.com