Initially, there are two polarizers. You can adjust the angle between those two polarizer by draging the small rectangle area. The intensity of light pass through the polarizer will vary as you change the angle between them.
No light will pass through when there are perpendicular to each other.
However, if you click the "middle piece" check box -- to add another polarizer between those two.
You will be able to see the image if you adjust the angle of the middle polarizer.