Plasma Rifle Overheat Glitch
From Halopedia, the Halo wiki
To perform the glitch, a player must obtain a Plasma Rifle or a Brute Plasma Rifle; they also need a second weapon.
All a player must do is fire the Rifle until it overheats, rapidly switch to their other weapon, and rapidly switch back. If done correctly, the player will be holding the Plasma Rifle up in the air. This will return to normal state after a few seconds.