Suggestions to Preston Briggs
This site was last updated on February 15, 2008.
Swing-arm shutoffs use a small swing arm with a spring on one end and a weight on the other. The spring tries to pull the arm to the right (in the picture above), pinching off the fuel line. But when the plane is flying in a circle, the centrifugal force on the weight pulls the arm towards the left, relieving the pinch and allowing the fuel to flow.
If the lines are cut, the plane flies off in a straight line, the centrifugal force is lost, and the spring pulls the arm back to the right, pinching off the fuel.
As an aid to starting, there's a tiny wire hook (barely visible to the right of the needle-valve knob) that's used to hold the arm in the open position. After take off, the centrifugal force will pull the arm to the left, releasing it from the hook.
This style of shutoff was quite popular for a few years, probably because they were readily available and easy to install. Recently however, it seems that various kinds of line-tension shutoff are taking over and some contests have prohibited swing-arm shutoffs.