When Playstation reads a disc, it looks for a special sequence on the disc that tells the Playstation “hey, this is a Playstation game. You should load it.”

That sequence is proprietary and isn’t on burned copies of games. This is anti-piracy protection, and makes sense from a monetary standpoint.

When you put Alien: Resurrection in the console, which has that sequence, the Playstation is told that “hey, this is a real Playstation game. You should load it.” The game loads, then you can put in the cheat, which tells the game to stop loading from the disc momentarily while another disc is loaded (think “please insert disc 2 from final fantasy”). At this point, you can pop in your burned copy of the game, then press a button to continue loading from disc, at which point the game tells the system “hey, this new guy is with me. Let him through”, and the Playstation loads the new game from the disc.