Body separation glitch
From Halopedia, the Halo wiki
The Chiropracter is a funny glitch in Halo 2 performable whenever desired. Basically, look down (as in at your feet), and spin around. If your look sensitivity is high enough, you will notice your torso spins faster than your lower body, and you can in fact turn your torso 360 degrees more than your legs. This will not affect your health or anything and can be done playing as either The Arbiter or Master Chief.