The launcher creates a local "tunnel" or emulated server environment on the user's PC. Instead of sending data to a remote Atari server, the game sends data to the launcher. This allows the game to proceed past the login screen and into the main menu without crashing.
Instead of patching the original .exe (which triggers anti-tamper), the launcher sits between your operating system and the game. It intercepts the failed network calls to Gamespy and reroutes them to a "localhost" (your own PC). It tricks TDU into thinking: test drive unlimited 1 offline launcher 13