From Halopedia, the Halo wiki
To do this method you must have one person in a Wraith (Player 1) and one person with an Energy Sword (Player 2) that is on the opposing team. Have Player 1 start moving backwards, then have Player 2 Sword cancel off the Player 1's Wraith. Player 2 will be propelled high into the air, easily allowing them to get to places like on top of the Zanzibar wheel. Another great level this can be done on is Headlong. This can also be achieved by having the Player 1 face Player 2, have Player 2 jump up and sword cancel and Player 1 boost at the same time. This secondary way also works with other vehicles (except the Scorpion tank).
Another way you can do this is by having Player 1 in the Wraith, and having Player 2 get an Energy Sword. Make sure that the Wraith has not been damaged. Player 2 must walk to the rear of the tank so that he is just in range with the sword. Sword Cancel, and you will go up a fairly decent height. For much better results, press A right after you press X.