This trick allows to use a car from one class (for example, Hill Climb) while racing in a different class. It can only be done online, and you must be the host of the game. Start your game with the game type that you want to race in (for example, Rally). Then, go to the game settings screen and select the class of the car you wish to use (for example, Hill Climb).
Then, press B to exit back to the lobby. Then, go to car selection screen and you may choose a Hill Climb car in a Rally race. Hill Climbs are much faster.