What it does:
1) Spawns Romero
2) Your current car has full immunities, invulnerable tires (won't pop), auto-unflip, is very heavy, and refills your health and armor.
3) In addition, while driving the Romero, you can press the submission button to turn on/off "Stealth mode": Gloomy weather, ignored by everyone, and slowed game speed. (One chime means On, three chimes is Off) All stats are removed once you leave your current car.

Note: While in Stealth mode you can still get a wanted level but you are not chased unless a)you turn off Stealth b)get out of Romero. Also, once you turn off Stealth or get out of Romero, the weather takes a short time to adjust back to normal.