This is some full-bore M.C. Escher stuff here, people. This cartridge contains a full NES and can play other NES cartridges. The cart uses a Famicom clone, sometimes called a Nintendo -On-A-Chip, and a few ports drilled into the plastic case.
From the DIY instructions:
One of the toughest things to find is the Chinese Power player unit (or Super joy thingy), which ever system you find, they all should work about the same. I was lucky enough to find mine at good will for 6 bucks… can’t beat that. Any ways, if you can’t find one locally then check on ebay. The only problem is that if you buy one off ebay then it will run you around 20 bucks. All the rest of the stuff you can get either from Radioshack, or once again on ebay. In my case I bought the 72 pin connector and the ports seperate, however you would probably be better off just buying a broken NES. My only problem was that I was kinda pressed for time, and couldn’t find one FOR CHEAP!!!!
In all, I’d say this is pretty darn wild.