When a Pelican-riding cutscene starts to play, hold down the "throw grenade" button. When the cinematic ends, you will immediately throw a grenade and remain inside of the Pelican, even though map scripting would normally force you to exit. (In Assault on the Control Room, you can use an alternate method; button-mash X (action button) or E (for Halo PC) to re-enter the Pelican as soon as you exit.) At this point, the Pelican will fly away on its scripted path, with you still inside. You may exit at any time; if you do not, the Pelican will eventually strike a kill barrier, resulting in your death.