" One of the most spectacular new features displayed in the E3 demo of Halo 2, was when Master Chief "jacked" ( or as Bungie likes to call it " boarded " ) a covenant Ghost hovercraft. While many thought this was simply an "cut-scene only" action, it is a new, and very in-depth feature that will be featured in the final game.

"Boarding will be implimented in both the single and multiplayer modes of the highly anticipated sequel. While the final implimentations have not been finalized ,here is how it will primarily work:

You can board any land based vehicle ( sorry no boarding of air-borne vehicles ). You can board both enemy AND friendly vehicles ( no more stopping the car when you are carrying the flag and need a ride to base.)
The player needs to face the vehicle he wants to board head-on, then wait for the screen to display a prompt at just the right moment (" press X to board Ghost") timing will be VERY crucial for boarding. Press the button too late or too early and you will be spartan road-rash. Attempting to board a vehicle from any other angle will still instantly kill the player.

Boarding an enemy vehicle:

Boarding vehicles comes with a price, your entire shield gets depleted ( this happens when you board both enemy and friendly vehicles ). This serves to prevent players from doing this move repeatedly during prolonged game sessions.
Once you successfully board the vehicle you have a limited amount of time to do the following *3* actions:

-1 Using your current weapon you can shoot the driver untill you kill him, or he decides to bail out.

-2 You can stick/drop a grenade inside/on the vehicle and then you make sure to jump off before you become ingulfed in the ensuing explosion.

-3 Last but not least:This is what most players will do, you can "melee" the driver out of the car, this is the fastest way to take control of the vehicle. The downside is that the driver remains mostly unharmed from this action and will be very purturbed that you just stole his ride."

( By the way, don't get c0cky and try to board a fully loaded enemy vehicle, all the passengers CAN and WILL shoot you).

"Boarding a friendly vehicle:

This works pretty much the same as boarding and enemy vehicles but with some slight but noteworthy differences.

You can only board multi-passenger friendly vehicles ( sorry no kicking a buddy off his ATV so you get all the flag-scoring glory).

Using the Warthog as an example, you can board only vehicles with a spare seat/ gunners seat open. Once on the hood of the car you must quickly choose wheter you would like to ride shotgun or gunner ( each choice will be mapped to an appropriate button press).

"The main benefit of boarding friendly vehicles is that it allows for faster getaways than the first game allowed.
Vehicles always get tripped with grenades when they stand still, or slow down dramaticly to pick up a passenger.So this will make for a much faster and dynamic multiplay experience, especially with CTF games."

Of course boarding enemy vehicles will keep them from bumrushing you for an easy kill, especially once you "slap" them out of their ride once."

"While all this information is very exciting, the game is still a work-in-progress and might be altered dramaticly from its current form. But I am certain we would all agree that it is certainly quite exciting what Bungie has in store for us. And imagine this is just ONE aspect of the many features the game will bring forth."

*Article from a Dutch Xbox gaming magazine, off-the-record interview with a bungie employee.*

Got it from GS forums.. (lol), but it's pretty cool