The NES has a very basic region protection method. It uses a 10NES Lockout chip inside the NES (16-pin DIP chip, often labelled 3193A for NTSC, 3195A for PAL-B and Korea, 3196A for Asia/Hong Kong or 3197A for PAL-A).

This chip checks when a cartridge loads if it returns a specific key that matches the expected value for that region.

Fortunately, bypassing it is incredibly simple. Cut pin 4 (Lock/Key) to disable the reset signal getting sent to the console (which is all it does if it detects a wrong region game to just reset the console.

Pin 4 is grounded for acting as a Lock, and 5V to act as the key. By cutting the pin it floats to ground and keeps the CIC is the "lock" mode which never resets the console (the Key mode is used to confirm and check against a lock and reset the console).

It can easily be restored by blobbing solder over the cut.

The only downside to this is something I feel is a myth and I have personally tested; that certain rare games that attempted to bypass this lockout chip by injecting a voltage spike to disable it, will overheat components in the NES if it is disabled.

Personally I find this unlikely, mostly sounding like a bit of mis-information from partial facts.