[Richard]

This subject has been addressed twice this year, but buried in otherwise titled posts. My purpose is to make a appropriately titled post for future gurus who could use the search engine to find the information. 

Here is a schematic of how the two valves are plumbed to the waste tank. 
   

The first thing to understand is that the grey water does not flow directly into the top of the waste tank. All grey water is plumbed into separate lines that eventually connect and flow into the waste tank through the bottom dump opening in the tank. Look at the diagram and you see how that happens. Compare the diagram to what you see in your wet bay. 

In normal operation, the manual valve is left open, and the air valve is closed. The grey water flows into the waste tank in this configuration. To dump the tank, the air valve is actuated. 

However, there is an alternative setup. Close the manual valve. Open the air actuated valve. In this configuration, black waste will accumulate in the tank, and grey water will continuously flow out the sewer drain. To dump the black tank requires opening the manual valve. 

I hope this helps in understanding a non intuitive setup.