The idea is to turn the plexiglassboxes so the container wil drop into the one below. Once you have logged, you put the container back in the tube in the top so it will fall back in the upper box. Its like an outdoors version of downfall
The creative geocache series, Maple Ridge Cryptex Caper is highlighted as an example of a very creative cache. Not only for their design and build of an actual cryptex, but also for the exceptional work down on the geocache listing page.