This cheese revolves around 2 things, the phantom design and the automatic errors

For the phantom design, for some reason, phantoms can only jumpscare you once per night, if they fail to do their attack, they can come again, but if they manage to do their attack, they won't appear again

For the automatic errors, for some reason, the ventilation error doesn't happen automatically in the Nightmare night (or night 6)

What this means is that if you get jumpscared by phantom Freddy and manage to fix the error before Springtrap comes, you can stay watching the door, and he can never kill you, since the automatic ventilation error doesn't happen and Phantom Freddy can't cause it since he can't come again, which results in the hardest challenge FnaF 3 can offer being completely cheesed