The "voltage spike" method caused a cat-and-mouse game between Nintendo and the unlicensed game companies, where Nintendo would make a new NES board revision and then the companies had to figure out a way around their new current protection. Some companies even published directions in their game manuals for how to modify the NES console to get around the protection: https://files.catbox.moe/9t03p6.png
> Nintendo would make a new NES board revision and then the companies had to figure out
I don’t understand how they can Do that and still maintain backwards compatibility with old cartridges?
The changes only broke (some) unlicensed cartridges, it didn't break cartridges that had a Nintendo manufactured protection chip.
You stick a diode on the reverse fry line, so sending -5v resets the console instead of the copy chip.
My father brought home unlicensed games from Hong Kong for my brother and I to play on the NES in the 80s. I recall after putting in the game and powering on the NES, the screen used to flash for a second, kind of like when the reset button was pressed. It's interesting to read that the negative voltage to evade the DRM may have caused that!