Grenade method
In this method you fall to the near right ledge where an exploding plasma grenade slows you up enough to let you direct yourself to where you can be cushioned below. Suitable only for NTSC/PC.
Full description
Let me start by saying that I haven't seen any footage of this method, nor have I been able to perform it from the original description. I'll do the best I can to describe it, but a few things are uncertain to me.
Get to the bottom of the V and face right. Throw a plasma grenade onto the ledge below (I'll say more on that shortly), pause for a moment, then run up the V a short way and head off almost parallel to the cliff, falling to arrive at the ledge just as the grenade is going off. If you timed things just right, the blast is non-fatal and it slows you up a bit, enough to now direct yourself to where you can be cushioned below as you skim off the ledge.
The exact nature of the cushioning is unclear to me but the originator - Gameovertime222 - said that you have to "hug the brown wall and slide to the rocks", and that "When you hit the rock you will slide down it and jump as soon as you can (if you keep sliding you will die for some reason)". Crouch as you land. I imagine that the "brown wall" is the darkish area of cliff face - though it doesn't look very brown to me.
There's also the question of grenade placement. Gameovertime222 referred to a carrot-like feature, which I suspect is what I've termed the chevron feature. He said to place the grenade in the "middle" of it, and to try to land right on it (i.e. on the grenade I think). I'm guessing that the timing has to be very precise for things to work.
Checkpoint advice
To make things as easy as possible for repeat attempts, get a delayed checkpoint when ready to throw the grenade.
Difficulty and system differences
Gameovertime222 was almost certainly using NTSC (he was in New Jersey), and said he could get down "every 2 or 3 tries and with at least yellow health". However, nobody else in the forum thread reported being able to manage the method, and I'd echo the opinion of one of the contributors that timing your arrival at the grenade is far from easy. There seems to be only a very narrow window there, and you could easily be killed. I suspect that most people would find the method very hard. It's always possible that Gameovertime222 overestimated his success rate of course, but I have no way of knowing.
Things may be even tougher with PAL, and perhaps actually impossible. PAL seems to give you less power of drift, which might make it difficult or even impossible to direct yourself to the cushioning area below the ledge, as seems to be the case with the right cliff base method (which I can't do for that very reason). But because the cushioning in this method remains unclear to me, I can't say for sure. In addition, the weaker traction available on PAL would presumably be a detriment to being able to stop from sliding off the ledge. I spent maybe an hour trying to get things to work (throwing the grenade into the chevron area), but had to give up. I have to say, I never once got any good feeling of being slowed by the grenade, even when I managed to arrive at what seemed the right moment.
History and links
This method was advertised by Gameovertime222 in the High Impact Halo forum on July 11th 2005, where it was accepted as being new. A video demonstration was provided four days later, and a further video was offered in September, advertised in the same thread. Sadly I've not been able to get hold of either (please get in touch if you can help). The question of whether he had NTSC remained unresolved, but he was "relatively sure" he had.