This lunar volvelle functions in the same way as many of the volvelles found in manuscripts. As the disk moves, the aperture reveals the seemingly changing shape of the Moon.
In many of these volvelles, the base disk shows the movement of the Sun during the year, and a smaller disk on top shows the movement of the Moon. The disks must be movable and separate as the solar and lunar cycles move at a different pace, yet must be aligned to calculate the date of Easter.
The video below shows how to use a lunar volvelle.