Warthog corkscrew jump
You know how the game plays a running video of Halo action if you leave it alone for a while before starting a game? And you know the bit where a Warthog gets blasted into a corkscrew trajectory by a Wraith? Well, you can launch yourself into that sort of spin if you've got a suitable ramp-like launching feature. The best launching feature is an unmanned Wraith (see Stunt driving off a Wraith in the level 5 fun section), but there are none available here. There are a few topographical features you can use however.
Marine plateau
In the rockslide survivor area there's a good place to do it as follows. Drive fast across where the Marine plateau meets the cliff face, with the plateau on your right. As you reach the hill that goes down, keep the right-side tyres on the plateau area that starts to go up, leaving the left-side tyres off the edge. And hey presto: one anticlockwise corkscrew jump coming up! It'll probably take a few tries before you get the technique right and manage to land on your wheels, but it's worth it.
Cliffside hilltop
Here's another example, though this is more a 'descent' than a jump. In the cliffside survivor area, you can drive up onto the plateau-like hill and then perform a corkscrew descent off the sharp edge on the valley side. See Driving off the edge in Cliffside hilltop fun.
Can you see out of that bloody visor?
When you're doing your corkscrew thing, be sure to carry a few Marines with you so they can amusingly question your driving skills when you almost kill them.