The Jandy valve has become the standard of the industry over the past 25 years. Several other manufacturers have similar diverter style valves that look and operate like the Jandy valve.
This valve has an internal diverter. There is a “block” that corresponds to the OFF indication on the handle. In the graphic, the off “block” is to the side, allowing the water to pass freely through the valve.
If the valve handle was turned 90 degrees, so that it was inline with the piping, it would then position the off “block” directly in front of of the ports, thus blocking the flow of water through the valve.
The basic operation of the off “block” is the same. If you block off one port, the water flows freely between the other two ports. If you block off no ports, the water is allowed to flow freely between all three ports.
In our article on valve settings, we show how these valves work in typical pool systems. We have diagrams that show the settings in various situations.