What was the fundamental design feature that made the Westinghouse system a fail-safe mechanism?

Answer

A loss of air pressure in the main line would automatically apply the brakes.

The system was engineered so that any intentional or accidental decrease in air pressure—such as a broken hose—caused the brakes to engage immediately.

What was the fundamental design feature that made the Westinghouse system a fail-safe mechanism?
inventiontrainbrake