Sword flying
From Halopedia, the Halo wiki
Summary
There are three ways to glitch involving the Energy Sword. In this article, 'R' is shorthand for the right trigger on your controller. 'X', 'Y', 'B' are the blue, yellow, and red buttons respectively.
Sword Cancelling
The first, sword canceling, is the act of lunging at a person without hitting them, by hitting R + X, effectively cancelling your attack. Another form of cancelling is B + X, which is useful for wall climbing (see below).
Sword Flying
The other form, commonly called rocket jumping or sword flying, is when you have both a Rocket Launcher and a sword, in either hand. First switch to the Rocket Launcher, then aim at a target so the crosshair turns red. Then quickly press X + Y + Right Trigger repeatedly until you "fly" at the target. This is useful for getting to otherwise un-reachable places high on maps, like Zanzibar and Headlong.
To get to really high places, you can use a combination of Sword Flying and at the top of the flight, using the Sword Cancel on an opponent to get to where they are.
In Halo 2 1.1, Halo 2 after the latest auto-update (as of 23/4/2005), Sword Flying has been disabled on Xbox Live, although lunging still works. Sword Cancels have not been disabled due to the auto-update.