Start a mission, then press the start button.
At this pause menu, hold down the left and right triggers, and the back button. (note, the back button is not "B", and it is not on the dpad. The back button in this cheat is the small one to the left of the xbox 360 guide button).
While holding down these buttons, press LB, LB, RB, X, RB, Y. Then release the two triggers and the back button. If done correctly, a tone will sound indicating that the cheat is active.