Ah yes - a piece of blu-tac and timing the swap just right.
For anyone else:
The lid of the PS had a small protusion which pressed down a button when the lid closed. The game could not be started with the lid open.
So a piece of blu-tac was used to depress that button.
The first thing the PS did when loading a game was to check somewhere on the game disc for some code. This code confirmed the disc was genuine so the trick was to put any genuine game in the drive, use the blu-tac, start the console and at a certain point, after the code had been read but before the game loaded and at that point pull the genuine disc and insert the pirate disc you had.
For anyone else:
The lid of the PS had a small protusion which pressed down a button when the lid closed. The game could not be started with the lid open. So a piece of blu-tac was used to depress that button.
The first thing the PS did when loading a game was to check somewhere on the game disc for some code. This code confirmed the disc was genuine so the trick was to put any genuine game in the drive, use the blu-tac, start the console and at a certain point, after the code had been read but before the game loaded and at that point pull the genuine disc and insert the pirate disc you had.