This cup has a built in siphon as shown in the cross section that only allows the cup to be filled to a certain level. Once more liquid is added above the top of the siphon all the liquid is drained through the base.
The same siphon principle is used in flush toilets. When the water level rises high enough in the bowl a siphon action sucks everything down the drain.
This cup was allegedly invented by the ancient Greek philosopher Pythagoras to keep his workers from drinking too much wine. If they filled their cup to full it would miraculously empty itself. I’m not sure I believe that story. You’d think they could drink 4 or 5 cups half full of wine or plug the hole in the bottom with their finger.
The author marked this model as their own original creation.