This page describes how it works. The gist is storing eggs in a container filled with a mixture of water and "water glass", a/k/a "liquid glass" or sodium silicate. If you do it correctly,
Fresh, unwashed eggs kept in a solution of water glass will remain good and useable for 4 to 6 months when properly collected and stored.Wikipedia lists a bunch of uses for sodium silicate, including home brewing and firefighting, so perhaps the material isn't hard to obtain.
(thanks to Jeffrey Viggiano)