How did region-locking on the SNES work?

USA readers may wonder why I was waiting for the release of a game already published. While Street Fighter II made it to the Super Famicom on June 10, 1992 in Japan and July 15, 1992 in North America, France had to wait until December 17, 1992 to get a PAL version.

As I waited, I saw ads in French magazines offering imported cartridges of my Holy Graal. To make them work on a European Super Nintendo, one had to buy an adapter. The combo cost almost as much as the console (595F + 199F vs 1290F).

Needless to say I couldn't afford it. But I always wondered how Nintendo seemingly controlled the regions and how tinkerers had managed to circumvent that protection.

A detailed look at how the 10NES sysyem worked.