It's like making your own peat pots. Each block stands alone in a tray of many blocks. When transplanting, the entire block goes in the ground. It's supposed to be much more efficient than using cell trays. I first learned about it from one of Eliot Coleman's books. Below are a few links that also explain it.
In my browsing on the web I did see directions for making your own block to put the soil in but it at first glance seems rather extensive - expensive - difficult. Would seem purchase of the gizmo if you have the $$ would be worth avoiding the DIY process.