The expansion port ties the cartridge port (on top, the one the game connects too) to the actual cartridge data interface (better terminology??).
If you remove the expansion port, on the bottom, you will have to solder the cartridge port pins to the outer holes where the expansion port was connected to get a data connection so that a game will play again.
I've tested this theory with the multimeter and much trial and error. If anyone believes differently, please explain.
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