The first two images are the only two pictures of the internal details of the tank that I am aware of. They come from page 16 of iRobot's Service Manual for the Scooba.

When the tank is in the horizontal position (first picture), the flap can hang down. When the tank is held vertically (second picture), the ball rolls to the edge of the area it is contained in and pushes the Flap Valve shut. The ball weighs about 78 grams so fairly heavy for its size.

The chamber above the flap valve is effectively a Dust Cyclone which we will examine in the next step...