See lower part of your body
From Halopedia, the Halo wiki
note this glitch can be done on the map Metropolis or The Great Journey.
On metropolis once you get up to the scarab part jump onto it then wait till it stops after it stops
go onto right or left back leg of the scarab walk into the body of the scarab and move back a little
then look down to see your legs moving without you.
(this glitch can be done on any difficulty)
This is basicaly the same with the great journey exept fly with the banshee onto it.